Cenozoic Incision History of the Little Colorado River: Its Role in Carving Grand Canyon and Onset of Rapid Incision in the Past GEOSPHERE; V

Home , Arizona transition zone, Bidahochi Formation, Black Mesa (Oklahoma), Chuska Mountains, Chuska Sandstone, Escudilla Mountain

Research Paper THEMED ISSUE: CRevolution 2: Origin and Evolution of the Colorado River System II

GEOSPHERE Cenozoic incision history of the Little Colorado River: Its role in carving Grand Canyon and onset of rapid incision in the past GEOSPHERE; v. 13, no. 1 ca. 2 Ma in the Colorado River System doi:10.1130/GES01304.1 K.E. Karlstrom1, L.J. Crossey1, E. Embid1, R. Crow2, M. Heizler3, R. Hereford2, L.S. Beard2, J.W. Ricketts4, S. Cather3, and S. Kelley3 18 figures; 1 table; 1 supplemental file 1Department of Earth and Planetary Sciences, University of New Mexico, MSC03-2040, University of New Mexico, Albuquerque, New Mexico 87131-0001, USA 2U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, Arizona 86001, USA 3New Mexico Bureau of Geology & Mineral Resources–New Mexico Institute of Mining & Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA CORRESPONDENCE: kek1@unm.edu 4Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, USA

CITATION: Karlstrom, K.E., Crossey, L.J., Embid, E., Crow, R., Heizler, M., Hereford, R., Beard, L.S., Ricketts, J.W., Cather, S., and Kelley, S., 2017, Ce- ABSTRACT for LCR and CR incision studies. Post–2 Ma differential incision magnitudes nozoic incision history of the Little Colorado River: (and rates) in the lower LCR and at the LCR-CR confluence were 280–320 m Its role in carving Grand Canyon and onset of rapid This paper documents a multi-stage incision and denudation history for (140–160 m/Ma), about three times greater than the 40–80 m (20–40 m/Ma) in incision in the past ca. 2 Ma in the Colorado River System: Geosphere, v. 13, no. 1, p. 49–81, doi:10 .1130 the Little Colorado River (LCR) region of the southwestern Colorado Plateau the LCR headwaters. /GES01304.1. over the past 70 Ma. The first two pulses of denudation are documented by The proposed mechanisms driving overall post–6 Ma differential inci- thermochronologic data. Differential Laramide cooling of samples on the sion of the LCR involve headwater uplift associated with the Hopi Buttes Received 14 December 2015 Mogollon Rim suggests carving of 70–30 Ma paleotopography by N- and and Springerville volcanic fields plus base-level fall caused by CR integration Revision received 10 August 2016 E-flowing rivers whose pathways were partly controlled by strike valleys at to the Gulf of California. A proposed mechanism to explain the accelerated Accepted 4 November 2016 Final version published online 10 January 2017 the base of retreating Cretaceous cliffs. A second pulse of denudation is docu- post–2 Ma differential incision in the central and lower LCR valley, but not in mented by apatite (U-Th)/He dates and thermal history models that indicate a the headwaters, involves mantle-driven epeirogenic uplift due to NE-migrat- broad LCR paleovalley was incised 25–15 Ma by an LCR paleoriver that flowed ing volcanism associated with the San Francisco volcanic field. Tectonically northwest and carved an East Kaibab paleovalley across the Kaibab uplift. driven differential surface uplift mechanisms were likely amplified by changes Lacustrine strata of the lower Bidahochi Formation were deposited 16– toward more erosive climate at ca. 6 Ma and ca. 2 Ma. 14 Ma in the LCR paleovalley in a closed basin playa or marsh with a valley center near the modern LCR. There is a hiatus in the depositional record in the LCR valley from 12 to 8 Ma followed by aggradation of the 8–6 Ma fluvial upper INTRODUCTION Bidahochi Formation. Interlayered 8–6 Ma maar basalts that interacted with groundwater mark local base level for upper Bidahochi fluvial deposits; this Little Colorado River (LCR) is one of the largest drainage basin area tribu- was also a time of increased groundwater flow to Hualapai Limestone at the taries to Colorado River (CR). As shown in Figure 1, its headwaters are in the western edge of the Colorado Plateau. The paleo–base level in the central LCR White Mountains and Springerville volcanic field at the southern edge of the valley remained stable (~1900 m modern elevation) from 16 to 6 Ma. Colorado Plateau. It flows in a broad valley across highly erodible Mesozoic The third pulse of regional incision and denudation, most recent and ongo- strata of the Holbrook and Winslow areas, enters a deep bedrock gorge in ing, started after integration of the Colorado River (CR) through Grand Canyon. Paleozoic rocks near Cameron, and has its confluence with the CR in Grand Thermochronology from Marble Canyon indicates that early CR integration Canyon 62 river miles below Lees Ferry (Stevens, 1983). Its modern river pro- took place across the Vermillion Cliffs at Lees Ferry after 6 Ma. The elevation of file is plotted along with the CR profile in Figure 2. The LCR profile departs from the paleoconfluence between the LCR and CR at 5–6 Ma is poorly constrained, a concave-up “equilibrium” profile in its lower and upper reaches; the lower but earliest CR integration is hypothesized to have reoccupied the East Kai- 75 km has a convex-up steepened reach in resistant Paleozoic rocks below a bab paleocanyon. In the upper LCR drainage, topographically inverted basalt knickpoint (LCR knickpoint). It also has sharp bedrock knickpoints including mesas have elevations and K-Ar dates indicating a transition from aggradation a ca. 20 ka basalt flow at Grand Falls (Duffield et al., 2006) and several knick- to incision ca. 6 Ma followed by semi-steady incision of 20–40 m/Ma. In the points in basalts in the headwater regions. lower LCR, incision accelerated to 120–170 m/Ma after 2 Ma as indicated by The overall goal of this study is to understand linkages between the Little For permission to copy, contact Copyright 40Ar/39Ar dating of basalt, ash-fall, and detrital sanidine. A 1.993 ± 0.002 Ma Colorado River and Colorado River systems and their influence on carving Permissions, GSA, or [email protected]. sanidine age for a tuff in the White Mesa alluvium provides a breakthrough Grand Canyon and to test models for landscape evolution in the region. We

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r San Juan Ri Rive 37° N Virgin ver ? V WM Kaibab 2B B San SV uplift 2A 4 Juan K 15A Figure 1. Map of the Grand Canyon region G C Basin 1 at ca. 15 Ma. Locations and extent of 15 Ma !5 S M NE !3 (1 Hopi paleolake-marsh is based on pres- (!4 Little Colorado 36° N (!2 ent elevations of the 15.8–13.6 Ma lower E 3 RB (! 7 (lacustrine) Bidahochi Formation; orange !N CA outcrops are upper (fluvial) Bidahochi For- !(F C 5 1900 m (! mation. Numbered incision points (yellow G 6 Pale (!A 10 stars) are keyed to Table 1; key thermo- !J Little Colora ori 11 M 15 ver chronology samples (red) are keyed to sec- ogollo 9 Highland tion DR-2 (see footnote 1); rim (green) and Mogollon Slop river (purple) thermochronologic samples n 12 W Hopi do Rive Paleo lake document East Kaibab paleocanyon (Karl- 35° N s (!B H 15 Ma 8 strom et al., 2014). Cross section location Springerville 13 r (!C for Figure 9 is shown. Red dashed line is volcanic eld (!I e axis of ca. 2 Ma White Mesa paleovalley. 14 18 Valencia surface (!L (!D 19 30 B—Black Mesa; C—Chuska Mountains; 00 SJ CA—Cameron; G—Gap; H—Holbrook; (!K 35 44 (! LCR thermochronology (!M (! M—Moenkopi Wash; MB—Mount Baldy; A samples H 16 34 RB—Red Butte basalt; SJ—St. Johns; SW SP SP—Springerville; SV—Shivwits basalt 34° N 1 LCR incision points on Grassy Mountain; V—Vermillion Cliffs; MB WM—White Mesa; W—Winslow. Contoured base of Salt River 1830 Bidahochi Fm. (m) 1900 N r 0525 0100 150200 Gila Rive km 33° N 114° W 113° W 112° W111° W110° W109° W

synthesize new and published data on the incisional history of the LCR region incision magnitudes. However, it is probable that both relative and absolute over the past 70 Ma and present the following data sets. (1) HeFTy modeling of elevations have changed due to differential surface uplift (Hunt, 1969, p. 113) low-temperature thermochronology data (from Flowers et al., 2008) provides resulting from a combination of changes in mantle buoyancy (Karlstrom et al., constraints on the 70–15 Ma denudation episodes. (2) Data on the Bidahochi 2008, 2011; Crow et al., 2014), fault dampened incision (Pederson et al., 2002; Formation and associated volcanic rocks (Dallegge et al., 2001; Dickinson, 2013) Karlstrom et al., 2007), and isostatic rebound due to differential erosion (Lazear provide a record of the 16–6 Ma time interval. (3) Incision rate data over the et al., 2013). past 7 Ma are derived from 40Ar/39Ar plus older K-Ar ages of basalt surfaces that form inverted topography and/or overlie gravel terraces in the LCR and its tributaries (Damon and Spencer, 2001; Holm, 2001a). (4) We also add 40Ar/39Ar ANNOTATED HISTORICAL BACKGROUND detrital-sanidine dates on elevated surfaces (Cooley et al., 1969) and paleoriver deposits (Hereford et al., 2016). (5) For the past 1 Ma, we report new 40Ar/39Ar Powell (1879) had little scientific comment on the LCR. Dutton (1882, p. 204) dates on basalts that entered the paleo-LCR channel and U-series dates on trav- thought it was as “old as the Colorado River itself” and viewed it as early ertine-cemented CR and LCR gravel terraces (Embid, 2009; Crow et al., 2014). Tertiary. Modern elevations are used to reference the relative landscape position Early workers interpreted the erosional history of the SW Colorado Pla- of constraint points, and the modern LCR river profile is used for estimating teau to have involved Colorado Plateau planation followed by entrenchment of

Elev. (m) Enlarged LCR prole in Fig. 14A

3000 y s on on t tion ny ny on y rr ash Cli olorado t Ca W wood Ca aibab Uplif ees Fe K L Little C Glen Ca ny Grand Junc Needles Grand 2500 Catarac Glen ternational Boundar

A In AM WM SP n HB 2000 RB Figure 2. Longitudinal profiles of the Little 2 Ma Colorado (LCR), Colorado, and San Juan San Jua rivers measured from 1: 24,000 U.S. Geo- logical Survey topographic maps along the river thalwag. Pink—Precambrian olorado basement; blue—Paleozoic strata; green— 1500 SV Little C o Mesozoic strata; yellow—Cenozoic depos- GF its. GF—Grand Falls; WMA—White Mesa olorad LCR C alluvium; projected elevations of selected knickpoint Pliocene (5–11 Ma) basalts are: AM— Anderson and McMillan Mesas; HB—Hopi 1000 Buttes; RB—Red Butte; SV—Shivwits; SP—Springerville.

500

0 0 500 1000 1500 2000 2500 (km from the Gulf of California)

the Colorado River and Grand Canyon incision (Dutton, 1882; Davis, 1901; see races averaging 137 and 213 m above the modern LCR) and Wupatki surfaces summary in Ranney, 2014). However, these various erosional styles and pro- (terraces averaging ~9, 15, 26, 54, and 76 m above the LCR). Cooley et al.’s (1969) cesses (regional denudation, cliff retreat, and canyon incision) are all entwined surfaces and terraces are listed in Table 1 with new age control from this paper. and have been driven by major tectonic base-level fall and/or headwater up- Wilson (in McKee et al., 1967), writing the consensus hypothesis of the 1964 lift episodes that reorganized drainages. These episodes were the 70–50 Ma Flagstaff meeting at the Museum of Northern Arizona, implicated the LCR to Laramide orogeny, 35–25 Ma ignimbrite flare-up and related caldera eruptions, reconcile two ideas. (1) Most workers envisioned a Miocene ancestral Colo 25–15 Ma extension of the Basin and Range, and 6–5 Ma integration of the rado River that originated in the Rocky Mountains and flowed to southern Colorado River (Karlstrom et al., 2011). Utah lakes and perhaps into northern Arizona (Hunt, 1969, his fig. 69). But, Cooley et al. (1969) applied and supported the peneplain concept that (2) the Muddy Creek Formation constraint indicated that the Colorado River stepped surfaces had formed during periods of erosion that beveled the land- could not have flowed across Grand Wash Cliffs in the location of the modern scape (Great Denudation of Dutton, 1882; Plateau Cycle of Davis, 1901; Rob- Grand Canyon until after 5–6 Ma (Blackwelder, 1934, p. 557; Longwell, 1946, inson, 1907; Gregory, 1947; Childs, 1948). These workers described numerous p. 829; Lucchitta, 1966). The Muddy Creek constraint has been questioned regional surfaces, some gravel or basalt capped. Highest surfaces that were (Wernicke, 2011, p. 1306; Flowers et al., 2012; Flowers and Farley, 2013) but interpreted to predate Grand Canyon incision were named the Valencia sur- remains supported by most workers (Lucchitta, 2013a; Karlstrom et al., 2014). face (pre-lower Bidahochi now known to be pre–16 Ma) and Hopi Buttes–Zuni To reconcile these apparent conflicts, McKee et al. (1967) routed an ancestral surface (pre-upper Bidahochi, hence pre–6–8 Ma). Surfaces interpreted to have Colorado River south from Utah, along the east side of the Kaibab uplift, to developed during Grand Canyon incision include the Black Point surfaces (ter- flow SE out the LCR valley, possibly to the Rio Grande drainage (their fig. 18).

TABLE 1. INCISION CONSTRAINTS FROM THE LITTLE COLORADO RIER LCR REGION River or Height m Sample Strath tributary above Incision Sample Age elevation elevation elevation CR, LCR, Age rate number Ma Name and location of constraint Latitude Longitude m m m or tributary Ma Error Dating method m/MaReferenceInterpretation and comments Lower Reach–Colorado River confluence to the Cameron area 1A 15–6 CR river mile RM 66.3; 4.3 RM 36.1271 –111.8223 820 1600 825 820 m 6 Large AHE AFT1371Approimate CR incision thermochron below LCR confluence CR on 01GC103 1B 2.68 Butte fault cave, RM 57 36.1271 –111.8223 1290 1030 445 CR; 2.68 Large -Pb 166–1002Maimum for CR; tributary incision to 260 trib Kwagunt Creek 100 m/Ma 1C 0.625 CR RM 57–66 36.1271 –111.8223 908 895 825 70 CR 0.623 0.1 -series 147–160 3Preferred CR incision strath-to- strath 160 m/Ma 2A Ca. 2White Mesa alluvium near The Gap 36.35976–111.37388 1700– 1700 1200– 375–425 20.02Ar-Ar DS 188–2134Poorly constrained Moenkopi Wash– 1740 1250 LCR confluence 2B Ca. 2White Mesa carbonates 36.47821 –110.99832 2023 24Constrains gradient of White Mesa paleovalley 3Ca. 2Blue Point tuff 35.988647 –110.99704 1817 1615– 272 1. 99 0.002 Ar-Ar sanidine 137 4Preferred tributary incision for Moenkopi 1780 236–308 119–155 Wash 4Ca. 1Tuff on Blue Canyon Plateau 36.13712–110.83907 1686 1520 122 Ca. 1Tephro122 4Preferred tributary incision for Moenkopi 118–126 118–126 Wash 5 0.89 Black Point basalt flow at LCR 35.669359 –111.34672 1489 1462 1290 1720.890.02Ar-Ar 1935, 8Preferred LCR incision rate 6A 0.87 Wukoki flow 35.529395 –111.282498 1405 1405 1316 1020.870.14K-Ar 1026Maimum LCR; Rice 1980 reported 97.7 m/Ma 6B 0.81 Woodhouse Mesa flow 35.504419–111.377159 1612 1570 1450 1200.812 0.059 K-Ar 1487Maimum tributary incision relative to Deadman Wash 6C nd Citadel flow, Antelope Wash 35.587874–111.402084 1590 1590 1305 285 nd 7Needs dating; height reported is to LCR 7A 0.34 Tappan Spring flow in LCR 35.881296 –111.442331 1308 1307 1250 57 0.342 0.006 Ar-Ar 1678Preferred LCR incision rate 7B 0.3 Shadow Mountain basalt, N of LCR 35.98551 –111.4357 1345 1295 50 0.28– 0.05– Ar-Ar 179–167 9Maimum tributary incision rate for 0.30 0.11 Moenkopi Wash Middle reach–Cameron to St. Johns 8A 16 Base of lower Bidahochi Formation, 1800 1480 320 15.840.052010 Echo Spring Mountain ash 14 m Winslow above base 8B 16 Base of lower Bidahochi Formation, 1770 1550 220 15.840.051411Base of lowest Bidahochi 1770 m Holbrook 8C 16 Base of lower Bidahochi Formation, 1800 1730 70 15.840.05412 LCR valley incision rate since 15.84 Ma St. Johns 9A 15.19Wood Chop D ash 35.42423 –110.04045 1810 1550 260 15.190.11Ar-Ar biotite1710LCR valley incision rate since 15.19 Ma 9B 13.7 Top of lacustrine Bidahochi, 35.31918–110.29225 1890 1470 420 13.710.08Ar-Ar sanidine 31 10 LCR valley incision rate since 13.7 Ma Holbrook 10 8.2 Tuff in Coliseum Maar 35.38333 –110.666 1558 1858 1450 408 8.20.18K-Ar 50 13 Maimum rate; 300 m below eruptive surface 10 7.84 Hopi Buttes diatreme 14 35.323 –110.32666 1886 2036 1468 568 7. 84 0.74 K-Ar 72 12 Maimum rate; 150 m below eruptive surface 10 7.84 Bidahochi basalt bomb 35.55633 –110.09 1987 1919 1465 454 7. 84 0.185 K-Ar 58 13 Bomb in Bidahochi Formation 10 7.82 Bidahochi dike 35.5605 –110.0928 1900 1915 1465 450 7. 82 0.18 K-Ar 58 13 Feeds overlying flow 15 m higher 10 7.71 unnamed Hopi Buttes mafic tuff 35.54635 –110.01662 1847 1847 1469 378 7. 71 0.06 K-Ar 49 10 Maimum rate 10 7.35 Bobcat Butte diatreme 35.47283 –110.4058 1873 1998 1455 543 7. 35 0.17 K-Ar 74 13 Maimum rate; 150 m below eruptive surface 10 7. 01 Na-Ah-Tee 2 basalt 35.47116 –110.188 1968 1968 1460 508 7. 01 0.16 K-Ar 81 13 Maimum rate, if surfaces sloped W to paleo-LCR 10 7.00 Hopi Buttes diatreme 35.54883 –110.1562 1964 1950 1460 490 70.24 K-Ar 70 13 Base of flow 1950 m from Google Earth 10 6.85 S of Roberts Mesa basalt 35.629 –110.099 1943 1920 1460 460 6.85 K-Ar 67 13 Maimum rate; 75 km from LCR 10 6.81 Petrified forest maar 35.0755 –109.802 1778 17601650 1106.81 K-Ar 16 13 Maimum rate; distant from LCR continued

TABLE 1. INCISION CONSTRAINTS FROM THE LITTLE COLORADO RIER LCR REGION continued River or Height m Sample Strath tributary above Incision Sample Age elevation elevation elevation CR, LCR, Age rate number Ma Name and location of constraint Latitude Longitude m m m or tributary Ma Error Dating method m/MaReferenceInterpretation and comments Middle reach–Cameron to St. Johns continued 10 6.62 Bobcat Butte diatreme 35.47166 –110.4037 1941 2041 1455 586 6.62 K-Ar 89 13 Maimum rate; 100 m below eruptive surface 10 6.62 Greasewood ash correlation age 35.48263 –110.0359 1907 1550 457 6.62 0.03 Tephro6910Maimum rate 10 6.60 Flat Mesa basalt 35.46333 –110.2447 1951 1950 1500 450 6.6 K-Ar 68 13 Maimum rate 10 6.56 Gray Mesa basalt 35.475 –110.1333 1916 1916 1500 4166.56 K-Ar 63 13 Maimum rate 10 6.55 Tesihim basalt 35.54333 –110.0995 1914 1913 1500 4136.55 K-Ar 63 13 Maimum rate 10 5.98 Deshgish basalt 35.52033 –110.0677 1946 1900 1500 400 5.98 K-Ar 67 13 Maimum rate; base of basalt 1900 m 10 7. 2Average Hope Butte eruptive 1928 1487 470 7. 2K-Ar 65 8 Maimum rate surface 11A7Top of upper Bidahochi Formation 2195 1480 7157 12 Highest outcrops slope toward LCR Winslow 11B7Top of upper Bidahochi Formation, 2255 1550 705 712uni surface of Cooley et al. 1969; St. Johns Gregory 1947 12A 6.39 Anderson Mesa and Walnut Creek 35.119911–111.579492 2180 1970 2106.390.3 K-Ar 33 7Maimum rate; Holm 2001 reported 49 m/Ma 12B 5.94 McMillan Mesa and Walnut Creek 35.152846 –111.610634 2165 2165 1970 1955.940.34 K-Ar 33 7Maimum rate; Holm 2001 reported 49 m/Ma 12C 0.86 Walnut Creek flow 35.122292 –111.61698 2156 2040 2022 18 0.859 0.055 K-Ar 21 7Approimate tributary incision rate for Walnut Creek 13A1.92 East Sunset Mountain upper flow 34.84642 –110.89843 2045 1945 1725 220 1. 92 0.09 K-Ar 1157Maimum tributary incision rate for E. Clear Creek 13B1.63 East Sunset Mountain lower flow 34.793358 –110.884456 1890 1865 1690 1751.630.23 K-Ar 1077Maimum tributary incision rate for E. Clear Creek 14 6.41 Chevelon Butte flow 34.690115–110.84864 2075 1750 325 6.41 K-Ar 51 14 Maimum tributary incision rate for E. Clear Creek 15A16alencia surface Black Mesa and Chuska uplift 600–760 16 Ca. 16 Ma; pre-lower Bidahochi 15 Composite erosion surface 15B10 6–8 Hopi Buttes and uni surface Hopi Buttes eruptive surface 300–450 7 6–8 Ma; pre-upper Bidahochi 15 Hopi Buttes eruptive surface of this study 15 Ca. 2 244–183 m LCR terrace 213 mEarly Black Point surface 1638 1425 2132 Blue Point Ash is 1. 99 Ma 15 Ca. 1.25 Ma, if 170 m/Ma steady L2A, C-1 incision 15 Ca. 1152–122 m LCR terrace 137 m Late Black Point surface 1562 1425 137Black Point flow at 151 m is 0.89 Ma 15 0.8–0.9 Ma based on Black Point flow L-2B, C-2 15 Ca. 0.6 91–61 m LCR terrace 76 mEarly Wupatki terrace 1501 1425 76 15 Ca. 0.45 Ma, if 170 m/Ma steady incision 15 Ca. 0.3 61–46 m LCR terrace 54 mWupatki terrace 1479 1425 54 Tappan flow at 57 m is 0.34 Ma 15 0.35 Ma based on Tappan flow 15 Ca. 0.1 30–23 m LCR terrace 26 mWupatki terrace 1451 1425 26 15 15 Ca. 50 ka 15 m LCR terrace 15 m Wupatki terrace 1440 1425 15 15 Ca. 0.1 Ma, if 170 m/Ma steady incision 15 Ca. 25 ka 9 m LCR terrace 9 mWupatki terrace 1434 1425 915 pper reach–St. Johns to Mount Baldy 16 6.80 Flat Top, SMC-5 34.0903 –109.2641 2450 2433 2134 299 6.80.02Ar-Ar 44 16 Overlies Fence Lake Formation upper Bidahochi Formation 17 6.52 5.31 Fissure vent basalt, 771 801C 34.5339 –109.5551 1925 1925 1725 200 5.31 0.01 K-Ar 31–38 17 Maimum 31–38 m/Ma LCR or 18–24 m/Ma tributary rate 18 6.03 S Fork campground, AWL-7-77 34.0547 –109.3946 2561 2561 2287 2746.030.43 K-Ar 45 18 Preferred; basal flow overlies river gravels 19 6.03 Black Mesa flow; 37 as dated 34.194 –109.011 2050 1805 245 6.03 0.02 Ar-Ar 41 16 Maimum LCR incision at N tip of vent runout continued

TABLE 1. INCISION CONSTRAINTS FROM THE LITTLE COLORADO RIER LCR REGION continued River or Height m Sample Strath tributary above Incision Sample Age elevation elevation elevation CR, LCR, Age rate number Ma Name and location of constraint Latitude Longitude m m m or tributary Ma Error Dating method m/MaReference Interpretation and comments pper reach–St. Johns to Mount Baldy continued 20 3.67 Coyote Wash flow, AWL 3-77 34.15027–109.17426 22231950 1815 1353.670.12K-Ar 37 18 Maimum LCR, or 33 m/Ma to Coyote Creek 21A 3.06 Springerville flow, AWL-42-74 34.11091 –109.3171321502150214553.06 0.08 K-Ar 219Flow at margin of modern flood plain 21B 2.94 Springerville flow, AWL-40-74 34.14478–109.27096 2153 2150 2125 25 2.94 0.14 K-Ar 9199 m/Ma relative to LCR at Springerville 22 2.37 Mesa Parada flow, NM 1160 34.45837 –109.0954 2000 20001750250 2.37 0.04 K-Ar 10516Flow is inset into Black Mesa 23 1.98 Escudilla Mountain flow, AKA82- 34.36054 –109.403171930 1930 1810 1201.980.6 K-Ar 60 20 Preferred LCR rate; overlies g2 194 gravels 24 1.67 AWL-5-77 34.2366 –109.346 20571940 1880 60 1. 67 0.09 K-Ar 36 18 Maimum LCR rate; height from below knickpoint 25 1.56 717MR 34.4608 –109.8399 1835 1715 1201.560.03 K-Ar 77 17 Preferred tributary rate; height from tip of runout 26 0.75 0.62 AWL 6-77 34.2534 –109.3534 1975 1870 1845 25 0.62 0.13 K-Ar 40 18 Normal magnetism, so 0.62 Ma ref 21 27 0.304 T21: EE07-81A 34.383686 –109.388851 1832 1827 1801 26 0.3040.013 -series 86 21 Preferred LCR rate 28 0.281 T23: EE-09-7 34.371775 –109.384805 1848 1805 43 0.2810.015 -series 15321Preferred LCR rate 29 0.255 T28: K04-SP-2, Lyman Lake 34.363628 –109.379623 1844 1841 1817 24 0.2550.006 -series 94 21 Preferred LCR rate 30 0.239 T19: EE07-84A 34.383686 –109.388851 1827 1827 1801 26 0.2390.003 -series 10921Preferred LCR rate 31 0.22 T26: K04-SP-5C 34.373108 –109.383268 1846 1833 1803 30 0.2240.003 -series 13421Preferred LCR rate 32 0.10 T40: EE06-LL 34.377572 –109.3901971829 1803 26 0.1010.0006 -series 25721Preferred short-term rate for LCR 33 0.097 T37: EE07-75-AR 34.359436 –109.369559 1847 1817 30 0.0970.014 -series 30921Preferred short-term rate for LCR 34 0.078 T38: EE06-LL 34.3703 –109.385769 1825 1804 21 0.0780.002 -series 26921Preferred short-term rate for LCR Headwater tributaries 35 6.73 TM-11-13-1 Teana Mesa Basalt 34.39961 –108.55907 223021402060806.730.18K-Ar 12 16 Maimum rate to Largo Creek 36 6.08 NM-1159 Mesa N of Blaines Lake 34.3169 –108.7635 2156 2156 2115 1356.080.04Ar-Ar 22 16 Maimum rate to Aua Fria Creek Blaines Lake 37 6.05 SMC-4 Cimarron Mesa basalt 34.25178 –108.8681 2230 22412115125 6.05 0.02 Ar-Ar 21 16 21 m/Ma to Aua Fria Creek; 30 to Largo Creek 38 6.03 SMC-1 Cow Springs, Black Mesa 34.1979 –108.9966 2360 23602170190 6.03 0.02 Ar-Ar 32 16 32 m/Ma Coyote Creek; 41 to LCR vent 39 5.20 SMC-2 basalt of Red Hill Draw 34.2057 –108.9256 2300 22802060120 5.20 0.03 Ar-Ar 23 16 Maimum to Aua Fria Creek 40 3.87 AWL-8-77 basalt, Nutrioso basin 34.0888 –109.2389 2251 22512150100 3.87 0.1 K-Ar 26 19 Maimum to Nutrioso Creek; overlies uemado Formation 41 3.67 AWL-3-77 basalt, Nutrioso basin 34.15085–109.1771 2225 22252110105 3.67 0.12 K-Ar 29 19 Maimum to Nutrioso Creek; overlies uemado Formation 42 2.46 SMC-3 basalt, Cow Springs basin 34.1691 –109.03558 2290 22902170120 2.46 0.04 Ar-Ar 49 16 Maimum to Coyote Creek 43 1.51 NM-1156 basalt, Red Hill basin 34.2704 –108.7192 2170 2166 2117 49 1. 51 0.01 Ar-Ar 32 16 Maimum to Aua Fria Creek Blaines Lake 44 0.97 NM-1157 basalt, Red Hill basin 34.29935 –108.76138 2155 2145 2112 33 0.97 0.014Ar-Ar 34 16 Maimum to Aua Fria Creek Blaines Lake References: 1. Lee et al., 2013; 2. Polyak et al., 2008; 3. Crow et al., 2014; 4. Hereford et al., 2016; 5. Haines and Bowles, 1976; 6. Rice, 1980;7. Holm, 2001; 8. This paper; 9. Conway et al., 1997;10. Dallegge et al., 2001; 11. Cooley and Akers, 1962; 12. Dickinson, 2013; 13. Damon and Spencer, 2001;14. Cather et al., 2008; 15. Cooley et al., 1969;16. McIntosh and Cather, 1994;17. Cooper et al., 1990;18. Laughlin et al., 1980;19. Laughlin et al., 1980; 20. Aubele et al., 1986; 21. Embid, 2009; 22. Condit et al., 1999. Ecept where ka is provided. Abbreviations: AHeapatite -Th/He; AFTapatite fission-track; CRColorado River; DSdetrital sanidine; ndnot determined; Tephrotephrochronology.

However, Rio Grande connections have not been supported by subsequent of Mesozoic strata until 25–15 Ma. Also, Marble Canyon was beneath 2–3 km of work nor have ancestral CR gravels been recognized in the Bidahochi Forma- strata until 6–4 Ma integration of the CR (Flowers et al., 2008; Lee et al., 2013; tion (Dallegge et al., 2001), nor in the 4 Ma to 55 Ma sedimentary record of the Karlstrom et al., 2014). Springerville area (Cather et al., 1994). The concept of a Miocene southerly exit In contrast to models for 70–30 Ma paleocanyons that followed the same of an ancestral CR to the Salt River was revived by Potochnik (2011) but was path as modern Grand Canyon (Wernicke, 2011; Flowers and Farley, 2012, 2013), not supported by Dickinson (2013) nor Karlstrom et al. (2014) based on a lack of others suggest that the Laramide drainage system involved rivers that flowed recognized CR deposits and on the evidence for a long-lived NE paleoslope for generally north across the Grand Canyon–Colorado Plateau region (Fig. 1, the rim gravels (Cooley and Davidson, 1968). This Mogollon slope persists McKee et al., 1967; Young, 2001; Karlstrom et al., 2013a, 2014). A fragmentary for tributaries to the LCR today (Fig. 1; Condit, 1991; Holm, 2001a). record of the 70–30 Ma landscape components and paleoriver deposits is pre- During lower Bidahochi time (now known to be 16–13 Ma; Dallegge et al., served as coarse gravels that rim the southern Colorado Plateau and contain 2001), the McKee et al. (1967) synthesis proposed that drainage from the unroofed basement lithologies from the Kingman and Mogollon uplifts. These S-flowing ancestral upper CR terminated in an internally drained Bidahochi gravels are referred to as the Rim gravels (Cooley and Davidson, 1963; Peirce, basin (their fig. 21). The term Hopi Lake was used for this lake by Williams 1984), Music Mountain Formation (Young, 2001), and Mogollon Rim Formation (1936), Repenning and Page (1956), Shoemaker et al. (1961), Sutton (1974), (Potochnik, 1989, 2001; Potochnik and Faulds, 1998). They are now known to Scarborough (2001), and Dickinson (2013). West of the Kaibab uplift, McKee be of disparate age. In the western Grand Canyon region, the Music Mountain et al. (1967) proposed that an independent W-flowing “Hualapai drainage sys- Formation (Young, 2008) is 65–55 Ma (Young and Hartman, 2014; Hill et al., tem” was established on the Hualapai plateau in pre-Miocene time (their fig. 2016) and is overlain by the Buck and Doe conglomerate that has a 24 Ma ash 20) with flow paths that were controlled by active faults and monoclines and in its upper parts (Young and Crow, 2014). In eastern Arizona and western New potential overall northward flow toward Claron and Uinta basins (Davis et al., Mexico, the Eagar Formation and the correlative Baca Formation are 50–40 Ma 2010; Dickinson et al., 2012; Karlstrom et al., 2014). (Cather, 2004). These are overlain by the 37–34 Ma Mogollon Rim Formation Integration of the modern Colorado River system was proposed by McKee on the Colorado Plateau, which is correlated with the 38–20 Ma Whitetail et al. (1967) to have involved post-early Pliocene drainage reversal of the pre- Formation in the Salt River Canyon (Potochnik, 2001). For Colorado Plateau viously S-flowing LCR as it was captured by a headwardly eroding Hualapai Rim gravels, local base level remained at similar elevation from 65 to 25 Ma, drainage system across the East Kaibab uplift (their fig. 22; but note that, in rivers were sourced in the contemporaneously uplifting Mogollon highlands, 1967, the Miocene–Pliocene boundary was thought to be ca. 12 Ma). Cooley regional paleoslopes were generally toward the north and east, and flow was and Akers (1961) and Hunt (1969) proposed a pre-Bidahochi LCR that crossed initially toward the retreating Cretaceous seaway (Potochnik, 1989, 2001). the Kaibab uplift south of Grand Canyon (as also supported by Karlstrom et al., The LCR valley has not been proposed as an important pathway for Lara- 2014 and in this paper), but Babenroth and Strahler (1945), Strahler (1948), mide and post-Laramide drainage (except Wernicke, 2011); instead most work- McKee et al. (1967), and Lucchitta (1988) envisioned breeching of the uplift in ers envision a broad alluvial plain with major rivers potentially reaching San Pliocene time by an ancestral CR, not a paleo-LCR. Proposed CR integration Juan and Baca basins (Potochnik, 2001; Cather, 2004) and/or Uinta Basin (Davis mechanisms have been diverse: headward erosion (McKee et al., 1967), lake et al., 2010; Dickinson et al., 2012). Paleocanyons developed 70–30 Ma such as a spillover from upstream lakes (Blackwelder, 1934; Scarborough, 2001), karst ~1-km-deep Music Mountain paleovalley (Young, 2008) and 1000–1400-m-deep piping (Hunt, 1969; Hill et al., 2008; Pederson, 2008; Hill and Polyak, 2010, 2014), Laramide Salt River paleocanyon (Potochnik and Faulds, 1998). Additional groundwater sapping (Crossey et al., 2015), and linkage of older paleocanyons thermochronologic evidence for km-scale of Laramide paleotopography is dis- (Karlstrom et al., 2014). Aspects of all of these mechanisms probably operated cussed below but is related to retreating cliffs of Mesozoic strata rather than during 6–5 Ma integration of the CR through the Grand Canyon region. to paleocanyons. Paleocene to Oligocene times involved aggradation in the Wernicke (2011, his fig. 9A) depicted the LCR as part of an E-flowing 70 Ma southern Colorado Plateau. Similar to the “turnaround” from an aggradational “California” river system that originated in the Sierra Nevada arc and flowed to an erosional landscape in the Rocky Mountains (McMillan et al., 2006), this through Grand Canyon, then southeast out the LCR to the San Juan Basin. transition took place in some locations after 25 Ma and after 10–6 Ma in the At 55 Ma, in his model, a divide formed in the middle of the modern LCR southern Colorado Plateau (Karlstrom et al., 2011). valley, and rivers flowed west in an “Arizona River” through a nearly com- The region of the confluence of the CR and LCR is of key importance for pletely carved Grand Canyon and east to the Eocene Baca basin and the Gulf river evolution models. Lucchitta et al. (2011, 2013b) proposed that a Miocene of Mexico. This model was supported by Flowers and Farley (2012, p. 3; 2013, (?) paleoriver that they named the Crooked Ridge paleoriver carved across p. 143c). However, there is no evidence for a paleo-LCR that had a role in either the East Kaibab uplift, a model now refuted by 40Ar/39Ar sanidine ages of 2 Ma of these proposed pre–50 Ma river pathways. Instead, as documented here on these paleoriver deposits (Hereford et al., 2016). Figure 3 is a photograph and by Flowers et al. (2008), thermochronology suggests that Moenkopi For- of the CR-LCR confluence looking east. The CR (green water) is dominantly mation rocks currently exposed in the LCR valley were buried beneath 2–3 km snowmelt from the Rockies; the LCR (turquoise blue water) is carbonic spring

The Gap

Big Creek LCR

Figure 3. Oblique view looking east at the confluence of the Little Colorado River (LCR; turquoise water) and Colorado River (CR; green water). CS—Cape Solitude. Grand Canyon is 1 km deep at this loca- 1600 m tion; CR elevation is 840 m; rim is 1830 m. ca. 6 Ma White lines show maximum depths of speculative LCR valley-bottom cross sec- 1200 m tions at 6 Ma that range from 1600 m to 1200 m elevation as loosely constrained 1145 m ca. 2 Ma CS-1830m by thermochronology sample 01GC- 103 (Lee et al., 2013; see Fig. DR-1.1 [see footnote 1]). Better constrained LCR can- 985 m ca. 1Ma yon bottoms are shown at 1145 m at ca. 2 Ma (near the top of the Redwall) and at 1240 m at ca. 1 Ma (below the base of the 825 m Redwall) assuming semi-steady 160 m/Ma incision rates near the confluence (Crow et al., 2014; this paper). Red lines at top of photo are the Gap (solid red line) and the nearest White Mesa alluvium (dashed red line). Photograph courtesy of Ted Grussing (copyrighted and used with permission).

CR 01GC-103 3 miles

Supplemental Materials for: water from Blue Springs (Crossey et al., 2009, 2015). To visualize the prob- Incision Methods and GIS Paleosurface Projections Cenozoic incision history of the Little Colorado River: its role in carving Grand Canyon and onset of rapid incision in the last ~2 Ma in the Colorado River System lem, the white dashed lines that bottom at 1600 and 1200 m elevation show Karl E. Karlstrom1, Laura J. Crossey1, Eileen Embid1, Ryan Crow2, Matthew Heizler3,Richard Hereford2, L. Sue Beard2, Jason Ricketts4, Steve Cather3, Shari Kelley3 Geosphere, 2016,DOI:10.1130/GES01304.1 speculative depths of the East Kaibab paleovalley at the ca. 6 Ma time of CR Bedrock incision estimates were made using strath heights of terrace grav- LIST OF DATA REPOSITORY (DR) ITEMS integration (Lee et al., 2013; Karlstrom et al., 2014). Alternatively, Cape Solitude els or base of basalt flows that had channel-like shape suggesting they flowed SECTION DR-1.Bedrock Incision Rate Data ……………………………….……………………….1 Figure DR-1.1 Thermal history model and paleodepth of sample 01GC103 (Lee et al., 2013) Figure DR-1.2 Incision rate for the 1.993 Ma Blue Mesa tuff relative to the Moenkopi Wash (CS, 1830 m) has been proposed as point of spillover of Hopi Lake to drive in the LCR or a tributary paleochannel. Incision rate data points are listed in Figure DR-1.3 Image of northern San Francisco volcanic field showing 0.89 Ma Black Point flow Figure DR-1.4 E-W cross section from Haines and Bowles (1976)showing drill hole RB No. 5 Figures DR-1.5, 1.6, 1.7 Photographs of incision points from travertine dated gravels CR integration (Scarborough, 2001). The 1145 m and 985 m lines show the Table 1. Description of bedrock incision rate calculation methods, individual Figure DR-1.8 Incision rates calculated from U-series ages of travertines Figure DR-1.9. Profiles of eastern tributaries to the LCR (above Holbrook Arizona) SECTIONDR-2. Thermochronology ……………………………………………………...... 18 elevation of the 2 and 1 Ma CR-LCR confluence based on steady incision rate incision points of this study, and their uncertainties are in section DR-1 of the Section DR-2 Thermochronology modeling Table DR-2.1 Table of key thermochronology samples data (Crow et al., 2014). The red line in the distance shows the Gap, a paleo- Supplemental Materials (see footnote 1). For samples distant from the LCR Figure DR-2.2 Age-elevation and eU-age data from LCR-region Figure DR-2.3 Youngest age –elevation plot for LCR samples Section DR-2.4 Geologic constraint boxes imposed on HeFTy models canyon-shaped “wind gap” in the Jurassic Navajo Sandstone along the Echo mainstem, we projected the height of the strath or flow base to the closest trib- Section DR-2.5 Sample by sample modeling details for samples A through N Figure DR-2.6 Comparison of models with and without theRim Gravel constraint box Figure DR-2.7 Comparison of models with and without the Bidahochiconstraint box Cliffs monocline, and the dashed red lines show the farthest west outcrops of utary and reported them as tributary incision rates. In the headwater regions, SECTION DR-3-40AR/39AR METHODS AND DATA ………………………………...... 41 Table DR-3.1. Analytical methods Table DR-3.2. 40Ar/39Ar basalt data the ca. 2 Ma White Mesa alluvium (Hereford et al., 2016; also referred to as the we digitized the elevation of bases and tops of basalt flows, gravel terraces Table DR-3.3. 40Ar/39Ar Sanidine analytical data SECTION DR-4. U-series data from travertines from the Springerville area ………………………..44 Crooked Ridge paleoriver deposits by Lucchitta et al., 2011, 2013b). Big Creek mapped by Sirrine (1958), and travertine straths and projected them perpen- is a dry tributary to the LCR. dicular to the river to estimate incision rates (Embid, 2009). 1Supplemental Materials, including bedrock incision rate data (section DR-1), thermochronology modeling (details for each sample; section DR-2), 40Ar/39Ar ana- Thermochronologic Modeling lytical data from basalts and sanidine (section DR‑3), METHODS and U-series data from travertines (section DR-4). This paper presents thermal history models of apatite (U-Th)/He (AHe) ther- Please visit http://dx .doi .org /10 .1130/GES01304 .S1 or the full-text article on www.gsapubs.org to view This paper integrates a variety of methodologies to understand the Ceno- mochronologic data from Flowers et al. (2008) using the program HeFTy 1.7.4 the Supplemental Materials. zoic landscape evolution of the LCR valley. (Ketcham , 2005). Constrained thermal paths are generated by inverse model-

Edge of Basin Basin and Range Figure 4. Simplified geologic map of the Little Colorado region (modified from Dickinson, 2013). Jurassic strata Major Laramide structures include the Hurricane, Kaibab, Echo, Monument, Defiance, and units undivided J and Range Province Apache uplifts. Labeled red dots are thermochronology samples (from Flowers et al., 2008) that Bidahochi Formation Triassic strata are modeled in this study and keyed to Table DR-2.1 (see footnote 1); black dots are thermochro- Normal fault Tbf T nology samples compiled in Karlstrom et al. (2014).

Monocline Nv Neogene volcanics Pr Permian strata Thermochronology TclTch Claron and Chuska Fm. Tg Rim Gravels & P Lower Paleozoic strata sample location WhiteTail Fm. ing using the radiation damage accumulation and annealing model diffusion Moenkopi Fm. samples Cretaceous strata model (RDAAM) (Shuster et al., 2006; Flowers et al., 2009). Section DR-2 in the !A K Pc Precambrian rocks modeled in this paper Supplemental Materials (see footnote 1) lists sample locations and informa- 114°W 113°W 112°W 111°W 110°W 109°WK tion. HeFTy takes predicted results from time-temperature (t-T) paths, com- Circle Cli s K pares them to observed data, and calculates a goodness of fit (GOF). Goodness Nv T Tcl of fit indicates the probability of failing the null hypothesis that the modeled Nv T t data and the observed data are statistically different. High values of GOF in- Nv Tcl Tcl dicate a high probability that the null hypothesis can be rejected and there is Pr K K San Juan hence a good match to the measured data. Goodness of fit values >0.05 are

Monumen River defined as acceptable agreement between modeled and observed data; values J J vier River >0.5 are regarded as good fits (Ketcham, 2005). A temperature of 15 ± 10 °C is P Se used to represent near-surface conditions, which encompass the 25 °C maxi- 37°N Colorado J mum surface temperature used for conversions of temperature to depth in this P T Ridge Kaibab paper as well as probably more realistic 10 °C surface temperatures. Hurricane Comb J Pr Echo Deance Black Tc 40Ar/39Ar Analytical Methods Pr Mesa h

Pc 40 39 K T Ar/ Ar dating was conducted by the New Mexico Geochronology Center. Analytical methods and results are reported in section DR-3 of the Supplemen-

36°N ash Tg W Pr !E tal Materials (see footnote 1). These build on the data presented in Hereford et al. (2016). !N !F Pr !G Uranium-series analysis of travertine was done on a Neptune inductively Little !A Nv Tbf coupled plasma mass spectrometer (ICPMS) in the Radiogenic Isotope Labora Tg !J Colorado Ri and P Nv tory at the University of New Mexico to establish absolute, high-resolution Nv Gr ages of periods of travertine deposition. Calculations used the following decay T 230 –6 234 ver constants: l of 9.1577 ± 0.0278 × 10 per year and l of 2.8263 ± 0.0057 × Puerco Tbf 10–6 per year (Cheng et al., 2000; note dates were not recalculated according to 35°N Nv !B West River Cheng et al., 2013). Errors reported are 2s confidence intervals. Two laboratory Tg Nv !C Mogollon Highlands !I blanks, analyzed for quality assurance, had measured values of <30 pg of 232Th Tg 238 230 232 !L !D Zuni and U. Ages were corrected using an initial Th/ Th activity ratio of 0.8 ± R. Pc Pr 0.4. Section DR-4 in the Supplemental Materials (see footnote 1) reports the K Pc !K Tg analytical data. !M !H LCR

Apache K

34°N Nv THERMOCHRONOLOGY RESULTS Nv Pc Tg A geologic map of the LCR valley is shown in Figure 4. The LCR region is Tg a broad strike valley cut mostly on the Triassic Moenkopi Formation. Precam- Km r brian basement is exposed in the Mogollon highlands to the southwest, and ³ 0525 0 100 150 200 Salt Rive progressively younger Phanerozoic strata are exposed to the north and north-

east. The now-eroded thickness of Jurassic and Cretaceous strata that once and a surface temperature range of 5–25 °C, these stratigraphic data suggest covered the LCR region is not well known, but estimates can be made based that lower Triassic rocks of the LCR valley would have resided at temperatures on nearby sections. As shown in Figure 5, Upper Cretaceous strata in southern of 80–110 °C at 85–70 Ma. This is in agreement with the ≥80 °C temperatures Utah, just north of Lees Ferry (Doelling and Willis, 2006), have a mean thick- suggested by Flowers et al. (2008) and with the 80–120 °C temperatures that ness of 1667 m (range 1194–2142 m). This is comparable to the thickness of best fit the thermochronologic data using the RDAAM model (this paper). 1785 m (1325–2245 m) for the Kaiparowits Plateau (Tindall et al., 2010). An However, this 3–3.5 km pre-Laramide depth-of-burial for the LCR samples incomplete section of 511 m (312–710 m) is preserved on Black Mesa (Nations indicated by the thermochronologic data contradicts Molenaar (1983), who et al., 1995). Jurassic strata are estimated as 1107 m thick (619–1492 m) in suggested that Cretaceous rocks tapered to a feather edge along the southern southern Utah (Doelling and Willis, 2006) to 1680 m (960–2400 m) exposed in Colorado Plateau margin. The original thickness of Jurassic strata in the study the Vermillion cliffs near Lees Ferry (Billingsley, 1989). Thus, based on strati- area is unknown and is complicated by a sub–Dakota Formation unconformity graphic thickness to the north, presently exposed lower Triassic rocks of the with progressively thinner earlier Mesozoic rocks to the south that indicates LCR valley may have been buried by up to 3–3.5 km of Mesozoic strata consist- that most or all of the Triassic and Jurassic section may have been denuded ing of 1.7 km of upper Cretaceous rock, 1.1–1.7 km of Jurassic rock, and several prior to ca. 94 Ma (Dickinson et al., 1989; Potochnik, 2001, his fig. 4), at least in hundred meters of Triassic strata. Assuming a geothermal gradient of 25 °C/km areas near the New Mexico border. The distribution of AHe thermochronology samples shown in Figure 1 includes 14 published AHe samples (Flowers et al., 2008). All but one sample Unit Cumulative is from the lower Moenkopi Formation of Triassic age, which has a deposi- thickness Rock Grand Staircase thickness tional age of 247–241 Ma (Dickinson and Gehrels, 2009). Sample M is from mean (range) name cli names (km) in m Permian strata (Fig. 4). The samples can be grouped into three cross-valley 3.5 NE-SW transects: Cameron transect closest to Grand Canyon (samples A, E, Tc 230 (170–290) Pink Claron Cli s F, G, J, and N), Winslow transect (samples B, I, and L), and Holbrook transect (samples C, D, H, K, and M). All three transects have samples that extend SW from near the LCR level up the ~0.5°–2°, NE-dipping Mogollon topographic 3.0 Kk (670) Kaiparowits slope (Fig. 1). The present LCR valley is broad and low relief, with total vertical relief between samples of 700 m over a lateral distance of 100 km (Fig. 1). Wah- The apatite grains were deposited as detrital sedimentary grains in the Meso 2.5 Kw 316 (122–510) Weap zoic sandstones and hence have crystallization ages older than the depositional age of the Moenkopi Formation. Based on analyzed detrital zircons from Figure 5. Stratigraphic column of the Grand Straight Canyon region (adapted from Doelling and Ks 441 (251–632) Cli s this formation (Anderson, 2006), >50% of the apatites are also likely to have Willis, 2006) from the areas just north of 2.0 Cretaceous ~ 1667 m formed in the Precambrian. AHe ages are all <0–70 Ma, suggesting that cooling Lees Ferry). Assuming a geothermal gradi- Gray ent of 25 °C/km, cliff retreat would be ex- Kt 190 (150–230) Tropic Cli s took place due to denudation following the 80–75 Ma onset of the Laramide pected to result in cooling of lower Triassic Kd 50 ( 01–100) orogeny (Goldstrand, 1994; Tindall et al., 2010). 1.5 Jm 140 (0–280) Morrison Moenkopi Formation rocks by ~40 °C due to Between four and ten apatite grains were analyzed from each sample and removal of the Cretaceous section (above Je 191 (91–290) Entrada yielded a wide range of AHe ages within individual samples (Table DR-2.1 in the Gray Cliffs) and ~30 °C due to removal Jc 179 (90–268) Carmel of the Jurassic plus upper Triassic sections the Supplemental Materials [see footnote 1]). This can explained by the RDAAM (above the Chocolate Cliffs). 1.0 Jn 405m White model (Shuster et al., 2006; Flowers et al., 2009), where crystals develop varying (290–520) Navajo Cli s amounts of lattice damage depending on their effective uranium concentration

Jurassic ~ 1107 m Jk 81 (58–104) Kayenta (eU = [U] + 0.235[Th]). Higher eU causes increased lattice damage, higher clo- Jw 112 (90–134) Wingate Vermilion 0.5 Cli s sure temperatures, and hence older ages. In this data set, the correlation be- TR 200m Chinle c (150–250) Shinarump Chocolate tween age and eU for the grains as a whole is weak (R2 = 0.13; Fig. DR-2.2B [see Cli s

~ 515 m 315m footnote 1]) but has a positive slope as predicted by the RDAAM model. TR Moenkopi R m T (300–330) 0 Flowers et al. (2008, their fig. 7) interpreted the data from these samples in Kaibab Plateau Permian terms of zones of diachronous cooling of the Kaibab surface below 45 °C, with 60–50 Ma for samples I–M high on the Mogollon slope above 1900 m elevation; 50–35 Ma for sample H; 28–18 Ma for samples A–G near the level of the LCR; Coal Recessive Resistant Limestone sandstone and sandstone and <6 Ma for the Marble Canyon region. These zones agree with the distribu- mudstone tion of the Rim gravels and the Bidahochi Formation.

80 Figure 6. Temperature-time paths generated by HeFTy for samples from the Holbrook transect; M. CP06-17 (2062 m) purple paths have goodness of fit (GOF) >0.5; green paths have GOF >0.05. Lower right diagram 70 shows weighted mean paths for each sample indicating progressive SW to NE cooling of samples, which also corresponds to progressive cooling from higher to lower elevation samples. 60 K. CP06-12 (2025 m) Complete modeling information, including GOF for each grain constraint relative to HeFTy’s best-fit model is in section DR-2 (see footnote 1). Age error bars are 20%. 50 H. CP06-10 (1957 m) 40

Th)/He Date (Ma) D. CP06-7 (1704 m) 30 C. CP06-6 (1628 m) To better evaluate the cooling history of each sample and implications for 20 denudation, we applied the RDAAM model using HeFTy to generate cooling

Apatite (U- Not used in 10 thermal modeling paths for each sample. Our modeling approach involved a number of steps that were applied systematically to all samples. Geologic constraint boxes 0 020406080 100 120 140 160180 were imposed on the models as summarized in section DR-2.4 and Figure DR- eU (ppm) 2.6 (see footnote 1). These included a Precambrian crystallization age of most 0 of the apatite grains (Anderson, 2006), early Triassic deposition (250–230 Ma), 20 heating to a pre-Laramide (90–80 Ma) temperature range of 40–140 °C due to 40 burial by an unknown thickness of Mesozoic strata, then cooling to modern ) 60 surface temperatures of 10–25 °C. The pre-Laramide portion of the path allows 80 for buildup of radiation damage in crystals that reside above or within the 100 AHe partial retention zone for extended time periods (Fox and Shuster, 2014). 120 Using these geologic constraints, and with a broad assigned error of ±20%

emperature (°C 140 T for corrected AHe ages, HeFTy time-temperature paths were generated that 160 predict most of the dated grains constraints according to the RDAAM model 180 M. CP06-17 D. CP06-7 (Table DR-2.1 [see footnote 1]) with a statistical GOF >50% (Ketcham, 2005). 200 0 Samples in proximity to Rim gravels must have been at near-surface tempera- 20 tures in the mid-Tertiary, and these were run with and without an appropriate 40 constraint box (Fig. DR-2.6 in the Supplemental Materials [see footnote 1]). ) 60 Similarly, samples in proximity to Lower Bidahochi Formation must have been 80 at near-surface temperatures by 16 Ma, and these were run with and without 100 this constraint box (Fig. DR-2.7 [see footnote 1]). 120 Details of the HeFTy modeling are described in section DR-2 with a page

emperature (°C 140 per sample in section DR-2.5 that shows the full time-temperature path (from T 160 the Precambrian), the post–100 Ma blow-up of the cooling history, the number 180 K. CP06-12 C. CP06-6 of path attempts needed to generate 100 good paths (GOF = 0.5), the number 200 of acceptable paths (GOF = 0.05), and the eU-age plot identifying the grains 0 that were modeled together. To evaluate how well the best-fit time-tempera- 20 ture path predicts the AHe data, we show its GOF for each grain constraint. All 40 time-temperature models are based on four to seven analyzed apatite grains; ) 60 C half of the samples generated successful HeFTy models using all analyzed M K H D 80 grains. For the others, one to three grains were excluded empirically in the 100 process of trying to generate acceptable and good-fit time-temperature paths 120 (Table DR-2.1 in the Supplemental Materials [see footnote 1]).

emperature (°C 140 T Figure 6 shows results of models for the southeastern (Holbrook) transect. 160 Samples C, D, H, K, and M extend from 1668 to 2025 m elevation, and the differ- 180 H. CP06-10 ent cooling paths are summarized in the bottom panel by the HeFTy-generated 200 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 weighted mean paths. Note that we show these smoother weighted mean paths Time (Ma) Time (Ma) rather than the more jagged best-fit paths for comparison, but the weighted

80 Figure 7. Thermal history models for samples from the Winslow transect. Lower right diagram shows weighted mean paths (from HeFTy) for each sample indicating progressive SW to NE 70 L. CP06-19 (2122 m) cooling of samples, which also corresponds to progressive cooling from higher to lower elevation samples. Complete modeling information, including goodness of fit (GOF) for each con- 60 I. CP06-20 (1957 m) straint relative to HeFTy’s best-fit model, is in section DR-2 (see footnote 1).

50 B. CP06-5 (1519 m) 40 Not used in

Th)/He Date (Ma) HeFTy models run with and without these geologic constraint boxes are very thermal modeling 30 similar. In this transect, lower elevation samples (also the samples farther northeast) remained hotter than higher elevation (southwest) samples until all paths 20 converge at ca. 18 Ma. The models show that near–river-level samples (C and D) Apatite (U- 10 resided at post-Laramide temperatures of 80–120 °C from 70 to 30 Ma. The mean-path temperature of river-level versus Mogollon rim samples between 50 0 020406080100 120 140 160 and 30 Ma varies by as much as 80 °C (Fig. 6, lower right), which far exceeds the eU (ppm) expected ~17 °C difference predicted by relative elevations. This suggests that 0 cooling took place from SW to NE (Flowers et al., 2008) and that the relatively Grains modeled: 7/8 20 Paths tried: 18076 sharp but diachronous cooling (M, K, and H versus C and D) suggests cooling Acceptable paths: 1358 was influenced strongly by cliff retreat. River-level samples (C and D) cooled 40 Good paths: 100 ) Avg. grain GOF: 0.58 from ~80–30 °C between 25 and 15 Ma, and converged with modeled tempera- 60 Grains modeled: 4/4 80 Paths tried: 2498 tures of higher elevation samples by 16 Ma, in agreement with the Bidahochi Acceptable paths: 183 100 Good paths: 100 geologic constraint that a broad paleovalley had formed by 15 Ma. Avg. grain GOF: 0.83 120 Figure 7 shows HeFTy-generated, time-temperature paths for the Winslow transect. Samples B, I, and L extend from 1519 m to 2122 m. Paths shown

emperature (°C 140 T for these samples were generated using most grains that modeled success- 160 Blue smooth path = weighted mean path Blue smooth path = weighted mean path fully together in HeFTy (B = 7/8, I = 7/7, L = 4/4; Table DR-2.1 [see footnote 1]). 180 Black jagged path = best- t path L. CP06-19 Black jagged path = best- t path B. CP06-5 200 We use the Bidahochi constraint for B and the Rim gravel constraint for I, but Figures DR-2.6 and -2.7 (see footnote 1) show that these geologic constraint 0 boxes do not markedly change the overall t-T paths. Similar to the Holbrook 20 transect, these samples show that the lower elevation (NE) river-level sample 40 L ) I (sample B; Fig. 7) resided at higher temperature than the higher elevation (SW) 60 samples. The best-fit path shows rapid cooling of the river-level sample at 80 Grains modeled: 7/7 B ca. 20 Ma, whereas the highest elevation sample had cooled through ~40 °C 100 Paths tried: 730 Acceptable paths: 96 before ca. 50 Ma. The intermediate elevation sample (I) shows an intermediate 120 Good paths: 100 Avg. grain GOF: 0.77 post-Laramide temperature of residence consistent with NE cliff retreat.

emperature (°C 140 T Figure 8 shows t-T paths for the northernmost (Cameron) transect, nearest 160 Blue smooth path = weighted mean path Grand Canyon. Cameron transect samples A, E, F, G, and J (plus N in section 180 Black jagged path = best- t path I. CP06-20 DR-2.5N) extend from 1417 m elevation (sample A) to part way up the north 200 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 side of the Gray Mountain uplift (sample E, 1910 m), which is a southern contin- Time (Ma) Time (Ma) uation of the East Kaibab uplift. Samples J and N are located on the south Kai- bab Plateau north of Flagstaff (Fig. 1). All of the samples in this transect were modeled in HeFTy with greater than half of the dated grains (A = 4/6, E = 5/8, mean path generally predicts the data less well than the best-fit t-T path (also F = 5/8, G = 5/9, J = 6/10; Table DR-2.1 [see footnote 1]). We use the Bidahochi shown in each model). All of these samples yielded multigrain “good” (GOF constraint for A, but the rest are shown without constraint boxes (Figs. DR-2.6 >0.5) time-temperature paths using a majority of apatite grains (C = 5/5, D = 3/4, and DR-2.7 [see footnote 1] show that these geologic constraint boxes do not H = 5/7, K = 5/5, M = 4/4; Table DR-2.1 [see footnote 1]). We show our preferred t-T markedly change the overall t-T paths). This transect shows the same pattern models: with the Bidahochi constraint imposed for C and D, and the Rim gravel as the other transects with samples J and N cooling in the Laramide and the constraint imposed for H, but Figures DR-2.6 and -2.7 (see footnote 1) show that rest cooling rapidly from 70 °C to near-surface temperatures after 30–20 Ma.

100 Figure 8. Thermal history models for samples from the Cameron transect. Lower right diagram 90 J. CP05-1 (1870 m) shows weighted mean paths (from HeFTy) for each sample. The pattern of progressive SW to NE cooling of samples, which also corresponds to progressive cooling from higher to lower 80 elevation samples is present, but more complex than the other transects. Complete model- G. CP05-2 (1832 m) ing information, including goodness of fit (GOF) for each constraint relative to HeFTy’s best-fit 70 model, is in section DR-2 (see footnote 1). 60 F. CP05-3A (1816 m) 50 h)/He Date (Ma)

-T E. CP05-7 (1704 m) 40

30 A. CP06-3B (1417 m) THERMOCHRONOLOGY INTERPRETATION: 70–50 Ma AND 20 25–16 Ma DENUDATION CONSTRAINTS Apatite (U Not used in 10 thermal modeling AHe thermochronology constraints on the denudational history of the 0 020406080100 120 140 southwest side of the LCR valley for the area of the Winslow and Holbrook eU (ppm) transects are shown on the left side of Figure 9. Geologic constraints on evo- 0 lution of 16–6 Ma paleosurfaces preserved by the Bidahochi Formation on the 20 northeast side of the LCR valley are shown on the right side. Each data set can 40 be compared and perhaps calibrated by the other. Shoemaker et al. (1961), ) 60 Grains modeled: 7/10 Cooley et al. (1969), and Cather et al. (2008) described a major denudational 80 Paths tried: 4338 Acceptable paths: 177 event that postdated the 35–27 Ma aggradation of the Chuska Sandstone in 100 Good paths: 100 Avg. grain GOF: 0.64 the Four Corners region and predated post–6 Ma CR integration (red arrow of 120 Grains modeled: 5/8 Paths tried: 6971 Fig. 9). Cather et al. (2008) estimated average rates of 119 m/Ma for this denu- Acceptable paths: 356 emperature (°C 140 T Good paths: 100 dational event. This denudation lasted until the 15.8–13.7 Ma deposition of the 160 Blue smooth path = weighted mean path Avg. grain GOF: 0.65 Blue smooth path = weighted mean path lacustrine lower Bidahochi Formation at modern elevations of 1770–1950 m 180 Black jagged path = best- t path Black jagged path = best- t path J. CP05-1 E. CP05-7 (orange in Fig. 9). In the headwater region of the LCR, this denudation pro- 200 0 duced a Late Oligocene to Early Miocene regional angular unconformity with 20 the exhumation of at least 1230 m of section by ca. 16 Ma (Cather et al., 2008). 40 This was later followed by a hiatus in the record (non-deposition?) from 13.7 to ) 60 8 Ma, then by 8–6 Ma aggradation of the upper (fluvial) Bidahochi Formation 80 (blue arrow in Fig. 9), then renewed incision to form the modern LCR valley in Grains modeled: 4/6 the past 6 Ma. 100 Grains modeled: 6/9 Paths tried: 46506 120 Paths tried: 8023 Acceptable paths: 2163 Thermochronology samples from the Winslow and Holbrook transects Acceptable paths: 608 Good paths: 100 mperature (°C 140 Good paths: 100 Avg. grain GOF: 0.72 near the cross section line are projected into a regional NE-SW cross section Te 160 Blue smooth path = weighted mean path Avg. grain GOF: 0.72 Blue smooth path = weighted mean path (30 times vertical exaggeration) in Figure 9. Stratigraphic thicknesses (from Black jagged path = best- t path Black jagged path = best- t path 180 G. CP05-2 A. CP06-3B Fig. 5) are shown along the left axis. Note these are maximum thicknesses as 200 Jurassic rocks may have been stripped prior to Late Cretaceous deposition. 0 Grains modeled: 5/8 Inferred paleosurfaces through time at 10 Ma intervals were constructed by 20 Paths tried: 7768 taking the weighted mean temperatures from the cooling paths (Figs. 6–8) and Acceptable paths: 244 40 J Good paths: 100 N assuming a 25 °C surface temperature and a 25 °C/km geothermal gradient; C) Avg. grain GOF: 0.73 A 60 these assumptions are inferred to produce minimal paleosurface elevations. 80 G E Thermochronology time-temperature paths suggest that cooling at the 100 F level of the currently exposed Triassic strata took place progressively from 120 SW to NE and that most samples underwent a period of relatively rapid cool- mperature (° 140 Te ing of 40–50 °C but at different times. The inferred paleosurfaces in Figure 9 160 Blue smooth path = weighted mean path suggest that this cooling may have been due to northward cliff retreat of 180 Black jagged path = best- t path F. CP05-3A ~1.7 km of upper Cretaceous strata (Gray Cliffs of the Grand Staircase) and 200 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 that Cretaceous strata remained above samples B and C at the LCR river level Time (Ma) Time (Ma) until after 20 Ma. Cooling due to retreat of Jurassic and Triassic cliffs (>1 km)

SW Maximum 50 Ma 50–20 Ma Paleo NE 70 Ma stratigraphic K & J cli s cli retreat surfaces 7 thickness Explanation: 7 MLK*IH DC Ma 70 b Bidahochi Fm. J Jurassic rocks 50 70 6 6 c Chuska sandstone TR Triassic rocks Figure 9. NE-SW cross section showing the r Rim Gravels P Paleozoic rocks denudation history of the Little Colorado K 5 5 River (LCR) valley from 35 Ma to present K Cretaceous rocks PC Precambrian rocks with geologically constrained paleosurfaces (right side; modified from Cather LCR 4 Paleovalley 4 et al., 2008). Thermochronologic samples tion (km) tion (left side) have letters keyed to Figure 1 J 30–15 Ma C and Table DR-2.1 (see footnote 1) and show

30 uska aggradation approximate heights of paleosurfaces va Ele max 27 Ma Ch 50 25 3 through time above each sample based 3 20 27–16 Ma incision TR 35 Ma base of Chuska Sandstone 100–145 on modeled mean t-T (time-tempera 30 15 11–6 Ma m/Ma ture) paths assuming 25 °C surface T, and aggradation face P 10 6 Ma eruptive sur 2 25 °C/km geothermal gradient. 2 M 5 90 L I mm/Ma < 6 Ma 1950 m hihighestg laklakee level K H incision D 1770 m base of the 16 Ma Bidahochiahochi FFmmm.. C B PC 30x vertical exageration 1 1 0 100 LCR 200 300 400 km from SW end of cross section

would have been another ~35 °C in areas such as Lees Ferry where the Juras- In addition to the progressive cooling by NE cliff retreat, a pulse of denu- sic Glen Canyon and San Rafael groups are present but would have been less dation is recorded by samples A, B, C, and D near river level in the LCR valley; (perhaps <25 °C) in the Cameron transect where the Glen Canyon Group was these remained >60 °C from 50 to 25 Ma, corresponding to preservation of 1–2 partially present, and near zero to the Winslow and Holbrook transects and St. of Mesozoic strata that still covered these rocks until ca. 25 Ma. All samples in Johns area, where upper Cretaceous strata rested on Triassic rocks at 90 Ma the LCR valley cooled to near-surface temperatures by 15 Ma, in agreement (Dickinson, 2013). with geologic constraints for a 16 Ma Lake Bidahochi basal surface. A broad Figure 10 shows our interpretation of the cliff retreat progression. For the paleovalley shape at 15 Ma is suggested by subequal elevations of the recon- Laramide time frame, all samples show a similar 90–80 Ma pre-Laramide structed 15 Ma paleosurface above the Mogollon slope to the southwest, with peak temperature of >80–100 °C that corresponds to burial by ~3–3.5 km of highlands of the Chuska-Defiance uplift to the northeast (Fig. 9). Hence we infer Mesozoic strata throughout the LCR valley and the Mogollon Rim. Similar that the NE rim of the LCR paleovalley at 25–15 Ma mimicked the modern Black thicknesses of upper Cretaceous strata may have extended SW across parts Mesa topography. of the Arizona transition zone as shown by the K(80) line in Figure 10 that was Additional thermochronologic evidence for such a paleovalley is seen in drawn southwest of samples with apatite fission-track ages of 60–90 Ma in Grand Canyon (Lee et al., 2013; Karlstrom et al., 2014) in rim and river rocks basement rocks (unlabeled red circles in Fig. 10). These samples suggest that (samples 1–5 of Fig. 1). These samples are separated vertically by 1.5 km and 3–4 km of combined Phanerozoic cover and eroded basement were removed resided at different post-Laramide temperatures compatible with a ~20 °C/km in these locations by 90–60 Ma. The highest elevation Moenkopi samples geothermal gradient, followed by convergence of cooling paths 25–15 Ma at- (J, K, L, M, and N) cooled rapidly between 70 and 50 Ma and are interpreted tributed to carving of the East Kaibab paleocanyon. The north rim of the East to record NE retreat of all of the Cretaceous plus any Jurassic section. We Kaibab paleocanyon is inferred to have had cliffs of Jurassic and some Creta- envision that 70–50 Ma paleorivers were bounded to the northeast by cliffs ceous rock at 15 Ma as shown by line K(15) in Figure 10. (K(50) line of Fig. 10) somewhat analogous to the Gray Cliffs section of the modern Grand Staircase. Samples H, I, F, and G remained at temperatures 40Ar/39Ar Basalt Dating Results of 60–80 °C after 50 Ma. The resulting post-Laramide (ca. 50 Ma) topography based on thermochronology is compatible with outcrops of 65–55 Ma Music 40Ar/39Ar spectra for four dated basalts are shown in Figure 11, and analyti Mountain Formation gravels near sample N and 50–35 Ma Rim Gravels near cal data are presented in section DR-3 (see footnote 1). All four of the dated samples J, H, I, K, and L. samples yielded reliable eruption ages between 9.17 Ma and 0.339 Ma.

Figure 10. Summary of inferred cliff retreat K (0) San JuanK Rive(0) history leading to the present locations River of the Grand Staircase cliffs. J(0)—base 37° N J(0) Virgin K (15) of the modern-day Jurassic section (Ver- J(0) million Cliffs); K(0)—base of the modern C r ? Kaibab W B Cretaceous section (Gray Cliffs). Tr(80) and uplift San J(80) represent the south edge of beveled Juan N-dipping Triassic and Jurassic strata, K G K (0) Basin respectively, at 80 Ma due to uplift of !5 the Cordilleran arc (Dickinson, 2013); this 3 (11 NE !3 !4 Little Colorado P erosional surface was overlapped south- 36° N 4 !2 !E wards by Upper Cretaceous strata. K(80) K (50) is speculative SW extent of Upper Creta- CA N !F ceous strata based on apatite fission-track (AFT) data. The unlabeled red dots are G !A aleoriver samples from Bryant et al. (1991) and !J Little C Music David Foster (personal commun., 2014) Mogollon Slopolora that yielded AFT ages of 60–90 Ma indicat- Mtn W Hopi ing burial by 3–4 km of rock in the Lara 35° N do Ri !B Paleo lake mide (e.g., ~0.5 km of basement, ~1 km H 15 Ma ver !C of Paleozoic, ~1.5 km of upper Cretaceous !I e strata?). K(50) is the interpretive position !L !D of the base of the Cretaceous section Mogollon SJ (Gray Cliffs) at ca. 50 Ma consistent with Highlands ! !K thermochronologic data for cooling of J, K, M !H K (15) L, M, and N to near-surface temperatures SW K (50) between 70 and 50 Ma due to erosional Jurrasic Cretaceous Tr (80) 34° N removal of Mesozoic strata. At 50 Ma, Conta Conta MB ct ct samples NE of the K(50) line record con- Cli Cli Salt River Eagar tinued residence at >80 °C until 25–15 Ma 60-90 Ma apatite K (80) ? suggesting that NE-retreat of Cretaceous ssion track ages cliffs drove 50–15 Ma progressive cooling LCR of I, F, E, B, D, C, and A. K(15) is inferred (!A thermochronology N to have been the north rim of the East samples 0525 0100 150200 Kaibab paleocanyon and the boundary of Gila River km 33° N Hopi Lake at 15 Ma. Other abbreviations as in Figure 1. 114° W 113° W 112° W111° W110° W109° W

Red Butte basalt is an isolated basalt-capped butte near the south rim of Point surface) by several workers (e.g., Billingsley, 2001; Holm, 2001a). The Grand Canyon that overlies Triassic rocks (RB of Figs. 1 and 2). A new 40Ar/39Ar Black Point flow originated in the San Francisco volcanic field and flowed into plateau age on this basalt is 9.17 ± 0.04 (Fig. 11), a refinement of the older K-Ar the LCR channel as evidenced by thick alluvial sediments encountered in a drill date of 9.73 ± 0.91 Ma (Billingsley, 2001). No gravels have been found beneath hole beneath the flow (section DR-1 [see footnote 1]; Haines and Bowles, 1976). this basalt. Tappan flow (location 7 of Fig. 1) was recognized by Colton (1937) to have A basalt at Grassy Mountain in the Shivwits volcanic field (SV of Fig. 1) was flowed >50 km north down Tappan Wash from the San Francisco volcanic field reported by Lucchitta and Jeanne (2001) to overly Precambrian clast conglom- and filled an earlier LCR river course (Rice, 1977). Our sample directly overlies erates on the west side of the mesa that were interpreted to be derived from LCR gravels. A new 40Ar/39Ar date on basalt from within a meter of the base of south of Grand Canyon. However, no far-traveled gravels were observed by us, the flow is 339 ± 5 ka (Fig. 11). and instead, the basalt overlies locally derived gravels. 40Ar/39Ar dating yielded a 5.47 ± 0.05 plateau age. 40Ar/39Ar Sanidine Dating Results from White Mesa Alluvium A new 40Ar/39Ar date of 891 ± 13 ka (Fig. 11) was derived for the Black Point flow (location 5 of Fig. 1); this agrees with an independent40 Ar/39Ar date from The White Mesa alluvium was considered to be Early Miocene (Crooked Hanson (2007) of 873 ± 8 ka. This age corrects an earlier inaccurate K-Ar age of Ridge paleoriver of Lucchitta et al., 2011, 2013b). Detrital-zircon (DZ) analysis 2.43 Ma that was applied to the Black Point flow (Cooley et al., 1969; late Black challenged this assertion because a single zircon age of 15 ± 4 Ma indicated a

much younger maximum depositional age (Fig. 12). 40Ar/39Ar dating of sanidine K06-RE0B-1, K07-SHIV-1, K05-LCR-TAP2, K05-LCR-BP was applied to an ash bed that is interbedded with White Mesa alluvium on the Moenkopi Plateau (location 3 of Fig. 1). This age was 1.99 ± 0.002 Ma (Hereford 10 et al., 2016). Similar ages on detrital sanidine (DS) of 2.02 ± 0.02 Ma at the base to 1.84 ± 0.05 Ma at the top of the section (Fig. 12) were found in the White Mesa alluvium of Crooked Ridge and White Mesa paleovalley, all at similar 8 9.17 ± 0.04 Ma (MSWD = 1.54) landscape positions. These dates indicate a ca. 2 Ma direct age for deposition of the ash and for the White Mesa paleoriver (Hereford et al., 2016). The full 40 39 6 Ar/ Ar K-feldspar age distribution (Fig. 12A) ranges between 1.8 and 1600 Ma and is dominated by grains younger than 40 Ma (n = 105). These grains are most certainly sanidine. Detrital grains between 40 and 250 Ma are most likely 4 5.47 ± 0.05 Ma (MSWD = 1.67) sanidine, and grains older than 500 Ma represent K-feldspar (microcline and orthoclase) from Precambrian basement. The Precambrian K-feldspar ages reflect the time that the basement source cooled through ~250–175 °C (Timmons Apparent age (Ma) 2 et al., 2005). The Mesozoic grain ages suggest reworking of the Dakota Sand- 0.891 ± 0.013 Ma (MSWD = 2.31) stone (Hereford et al., 2016). In addition to the breakthrough about the young age for the White Mesa 0 0.339 ± 0.005 Ma (MSWD = 2.37) alluvium, the DS data provide new insights about provenance. The detrital- sanidine (DS) data for Cenozoic crystals show a 100-fold improvement in pre- 0 10 20 30 40 50 60 70 80 90 100 cision of DS single-crystal ages compared to those of DZ (Fig. 12B). This al- Cumulative %39Ar Released lows very high precision correlation of individual grains to specific ignimbrite eruptions. Many DS nodes are indistinguishable in age and K-Ca ratios from A BA sanidine from San Juan volcanic field (SJVF) ignimbrites (Fig. 12C). For example, one population of White Mesa alluvium grains yields a weighted mean age of 28.209 ± 0.013 Ma and corresponding K/Ca values near 70; these values are distinctive of the 5000 km3 Fish Canyon Tuff sanidine, one of the world’s largest single eruptions, sourced from the La Garita caldera of the San Juan Mountains (Lipman and McIntosh, 2008). The next oldest population of DS grains is 28.685 ± 0.021 Ma with a K/Ca of ~20 and correlates to Blue Mesa Tuff also from SJVF. None of the dated grains yielded ages between 28.3 and 28.6 Ma, a major pulse of volcanism in New Mexico’s Mogollon Datil volcanic field (McIntosh et al., 1992). Correlating prominent age peaks as well as temporal gaps in the White Mesa DS data indicate that the ultimate primary source CDfor Oligocene detritus in the White Mesa alluvium was the San Juan Moun- tains of Colorado rather than volcanic fields in New Mexico. Many of the White Mesa DS age peaks are currently uncorrelated. For in- stance, the young grains near 2 Ma were first thought to correlate to the large Huckleberry Ridge eruption of Yellowstone. However, upon closer comparison to manuscripts by Ellis et al. (2012) and Singer et al. (2014), the DS peak at 2.0 ± 0.02 Ma (normalized to the same standard ages) is distinguishably younger than the published 2.15–2.10 Ma Huckleberry Ridge ages. The DS data are chemically distinct as well: K/Ca for this DS peak is ~100, whereas Huckleberry Ridge is ~10. Similarly, the 10–5 Ma grains are currently uncorrelated ages but Figure 11. 40Ar/39Ar release spectra, plateau ages, and photos of newly dated basalts in the Little Colorado River would be compatible with northern Great Basin sources (Perkins et al., 1998; (LCR) region. See Figure 1, Table 1, and section DR-1 (see footnote 1) for locations and section DR-3 (see foot- Anders et al., 2009; Best et al., 2013). note 1) for analytical data. (A) 9.17 Ma Red Butte flow near south rim of Grand Canyon; (B) 5.47 Ma Shivwits basalt on Grassy Mountain north of Grand Canyon; (C) 891 ka Black Point basalt in the LCR valley; (D) 339 ka Precise DS data near 18.8 Ma are shown in Figure 12D. Hereford et al. Tappan basalt in the LCR valley. (2016) suggested that these grains could have originated either from the Peach

250

A a B 150

N K/C 50 All Zircon 2.02 ± 0.02 Ma N Youngest DZ 15±4 Ma Figure 12. Detrital-sanidine (DS) and detrital- y 1.84 ± 0.05 Ma zircon (DZ) data from the White Mesa allu Apache Leap N vium. (A) Zircon age spectrum (green) show- 18.78±0.03 Ma ing dominant populations of Precambrian, Paleozoic, and Mesozoic grains; sanidine and microcline ages (blue) are dominated by younger volcanic grains as well 800–1200 Ma cooling ages for regional K-feldspars. (B) DS data from 0 to 20 Ma give a maximum depo N sitional age for two stratigraphic positions between 1.84 and 2.02 Ma, whereas youngest Relative probabilit DZ is 15 ± 4 Ma; based on ultrahigh precision 0 2 4 6 8 10 12 14 16 18 20 data, the node at 18.78 ± 0.03 Ma and K/Ca near 60 is sanidine from the Apache Leap tuff. Age (Ma) (C) High resolution of DS data between 27 and 32 Ma define very discrete nodes with known correlation to the San Juan volcanic field. 250 D (D) ARGUS VI multicollector mass spectrom- C 150 eter single-crystal laser fusion sanidine geochronology comparison of Apache Leap tuff K/Ca 50 to Peach Springs tuff in relation to previous data collected on single-collector MAP-215‑50 N mass spectrometer. Even at ultrahigh precision, the two eruptions are indistinguishable in age; however, the distinctive K/Ca values DZ Fish Canyon DS provided by the improved data demonstrate DS that the three DS crystals from White Mesa 28.209± Blue Mesa alluvium sediments were sourced from the 0.013 Ma 28.685± Apache Leap tuff. Thus, the White Mesa allu vium contains detritus ultimately derived 0.021 Ma from both the San Juan volcanic field and the Superstitions Mountain ignimbrite eruptions, e probability but both could be reworked. Relativ 27 28 29 30 31 32 Age (Ma)

Springs tuff near Kingman, Arizona, or the Apache Leap tuff near Phoenix, the K/Ca data for White Mesa DS grains (K/Ca = 45–55) are a better match to the Arizona. Hereford et al. (2016) could not distinguish a Peach Springs versus higher values from the Apache Leap tuff (K/Ca = 30–55) indicating Apache Leap Apache Leap source because the published 40Ar/39Ar geochronology of these tuff was a source for White Mesa sanidine. This might support the conclusions tuffs was too imprecise (i.e., existing geochronology data; McIntosh and Fer- of Potochnik (2011) for northerly flow of a paleo–Salt River into the proto–Little guson, 1998; Ferguson et al., 2013) could not decipher an age difference be- Colorado River basin. However, wind transport of sanidine detritus is likely tween the tuffs). Also, the scatter in the published K/Ca data prevented chem- and would preclude firm conclusions about fluvial connections between White ical diagnosis (Fig. 12D). We have re-dated both tuffs with the ultra-precise Mesa alluvium and the areas south of the Mogollon Rim. Thus, the White Mesa ARGUS VI mass spectrometer and find that, even at ca. 5 ka precision, we alluvium contains detritus ultimately derived from both the San Juan volcanic cannot determine an age difference between these tuffs (Fig. 12D). However, field and the Superstitions Mountain ignimbrite eruptions.

RESULTS Triassic strata. By 15–6 Ma, Jurassic (Vermillion Cliffs–type) and Cretaceous (Gray Cliffs–type) escarpments had retreated north of Grand Canyon and east 16–6 Ma Geologic Incision Constraints of Bidahochi Formation outcrops.

Reversal of the NE- and E-flowing post-Laramide rivers to SW-flowing Post–6 Ma Geologic Incision Constraints streams along the Mogollon Rim in the LCR headwater region took place after 18.6 (and perhaps after 14.5 Ma) for the Salt River as a result of Basin and Range This section presents new incision control points for the LCR in the context extension and collapse of the Mogollon highlands (Faulds; 1986; Potochnik of a synthesis of all published incision constraints (Table 1 and section DR-1 and Faulds, 1998; Potochnik, 2001). This drainage reversal may have led to the [see footnote 1]). Figure 14A shows the longitudinal profile of the LCR and 16–6 Ma period of internal drainage in the LCR valley (Bidahochi Formation). some important tributaries, the bedrock that underlies the LCR profile, and From 16 to 6 Ma, the LCR paleovalley was a shallow Bidahochi depositional the rim of the deeply incised LCR canyon as it enters Grand Canyon at the left basin (Fig. 1; McIntosh and Cather, 1994; Scarborough, 2001) with a relatively edge of the diagram. Red dots represent incision constraints projected into the stable local base level. The lower Bidahochi lacustrine facies was deposited from profile keyed to Table 1. 15.84 to 13.71 Ma as constrained by interbedded ash layers; it reached a total Modern surface water of the LCR is from several sources. In the lower stratigraphic thickness of ~100 m (Dallegge et al., 2001). The depositional en- reach, Blue Springs is a spring source of perennial flow (~220 cubic feet per vironment is interpreted to be a shallow playa or marsh, with no evidence that second; Crossey et al., 2009) for the LCR that comes from the Redwall-Muav Hopi paleolake was ever a deep lake (Dickinson, 2013). Figure 1 shows structure aquifer. In the central reach, flow is ephemeral and controlled by spring runoff contours of the base of the Bidahochi Formation (based on outcrops and drill- and flash floods. In the upper reaches, surface flow is from spring sources and hole elevations) that outline the depocenter (Fig. 1; Cooley and Akers, 1961). snowmelt from the White Mountains. Using the landscape position of dated ash beds as long-term incision and denudation constraints, the post–16 Ma long-term average bedrock incision CR-LCR Confluence rate for the LCR valley has been very slow (17–31 m/Ma for points 9A and 9B of Table 1). These slow, post–16 Ma averaged rates indicate that 16–6 Ma aggra- At the CR-LCR confluence, estimating total magnitude of post–6 Ma incision dation has been only slightly exceeded by post–6 Ma incision. From 16 to 6 Ma, depends on the depth of the postulated 25–15 Ma East Kaibab paleocanyon, aggradation of sediments in the central reaches filled paleovalleys with fine- which would have formed the initial base level for integration of the CR with grained lacustrine deposits in the lower Bidahochi Formation and with coarser the paleo–LCR valley 5–6 Ma. Scarborough (2001) used an elevation of 1830 m fluvial upper Bidahochi Formation (Dallegge et al., 2001; Dickinson, 2013). In and envisioned that spillover of Hopi Lake at Cape Solitude initiated downward the headwater region, gravels were deposited until ca. 6–4 Ma (McIntosh and CR integration. However, modeling of thermochronologic data from near the Cather, 1994), suggesting a generally aggradational setting from 16 to 6 Ma. confluence suggests that river-level rocks had cooled to temperatures of 20– Figure 13 shows a series of erosional surfaces that record denudation of 30 °C by 10 Ma and that East Kaibab paleocanyon may have been cut to below the LCR valley generalized from mapping of Cooley et al. (1969), with new the level of the Kaibab limestone in Eastern Grand Canyon (Lee et al., 2013). geochronologic control from this paper. Surfaces high in the landscape in the Dickinson (2013) rejected the spillover model based on conclusions that Hopi Chuska Mountains and on Black Mesa were named the Valencia surface and Lake was not at the correct elevation and was never a deep lake, but Crossey interpreted to be the result of 26–16 Ma (post–Chuska Sandstone) denuda- et al. (2015) cited groundwater sapping as a possible additional mechanism for tion (Cooley et al., 1969). Cooley et al. (1969) also “correlated” the sub-basalt downward integration. A speculative range of minimum elevations (maximum “surface” at Red Butte on the south rim of Grand Canyon as Valencia surface paleocanyon depths) for the ca. 6 Ma confluence is 1200–1600 m (Fig. 3). Lee (Cooley and Akers, 1961). In addition to the Red Butte basalt, other locations in et al. (2013, their fig. 9E) showed a preferred elevation of ~1600 m (Fig. DR-1.1 the region show 10–6 Ma basalts overlying Triassic strata (Fig. 3): Flagstaff area [see footnote 1]), which we use for paleoprofile reconstruction in Figure 14A (Holm, 2001a), Red Butte, Anderson, and McMillan mesas on the Mogollon (point 1A of Table 1). slope (Holm, 2001b), and a 6.41 Ma basalt flow at Chevelon Butte (Cather et al., Positions of the 4–2 Ma paleoprofiles in eastern Grand Canyon are not 2008). At Long Point in Arizona, a 6.7 Ma basalt flow overlies the Music Moun- constrained unless one assumes semi-steady incision (e.g., Karlstrom tain Formation at ~1800 m elevation suggesting no significant degradation et al., 2008). Polyak et al. (2008) reported a 2.68 Ma age from a cave speleo there between 65 and 7 Ma. Similarly, 7–5 Ma basalts of the northern Shivwits them in Kwagunt Creek that is ~5 river miles upstream from the conflu- Plateau on the North Rim of Grand Canyon overlie Triassic rocks (Lucchitta ence. The sample is at an elevation of ~1290 m, 260 m above the Kwagunt and Jeanne, 2001). Although not a single “surface or peneplain,” the regional Creek tributary and 445 m above the river. They assumed the water table distribution of 10–6 Ma basalts overlying Triassic strata indicate a relatively was flat and that the speleothem formed during downward passage widespread, locally aggrading, moderate relief landscape carved into lower of the water table that tracked river level as CR-incised Grand Canyon.

! Cortez

Navajo ! Mountain Page 37° N ! Shiprock Kayenta ! !

G Figure 13. Little Colorado River region river terraces and “surfaces.” These were mapped by Cooley et al. (1969, Plate 3) BC based on relative landscape position and were interpreted by them to represent regionally correlative former incisional surfaces. New age constraints on river

36° N MW terraces (Table 1) are 0.89 Ma Black Point basalt (blue star), and ca. 0.34 Ma Tappan Cameron flow (purple star). Surfaces that may be pediments graded to terraces are ca. 2 Ma White Mesa alluvium (red stars) and ca. 1 Ma Bishop tuff (green star). Bidahochi Gallup Formation undivided (orange) is from ! the Arizona and New Mexico state geologic maps; base of ca. 16 Ma lacustrine Bidahochi Formation (yellow line) and ca. 6–8 Ma Hopi Buttes eruptive surface Flagstaﬀ (black dashed line) are from Figure 1. ! Approx. Geologic Cooley et al. This paper (modified from ! age (Ma) unit/surface (1969) surfaces Cooley et al. 1969)Winslow! 0.3 L-3, L3A, C3 to C5 Wupatki terraces Zuni Pueblo 35° N ! 0.89 L-2B, L2C, C2 Black Point terrace Holbrook ~1.0 L-2B, L2C, C2 Blue Canyon Plateau ~2 WMA L2A, C1 White Mesa alluvium Moenkopi Plateau N ~6 Bidahochi undivided St. Johns ! ~6–8 Hopi Buttes eruptive surface

14–16 base of lacustrine Bidahochi Payson Show Low ! ! >16 –< 25 L-1 Valencia surface Springerville ! 112°W 110°W

LCR-CR Conuence Little Colorado River (LCR) Prole Figure 14. (A) Profile of the Little Colorado River (LCR) showing interpreted 16–1 Ma paleo 3000 5 Ma Incisional Paleopro le WM White Mesa paleoriver surfaces of the LCR region. Red dots are incision constraints (Table 1): 15.8–13.7 Ma Hopi 2900 A A Lake (Bidahochi lacustrine facies) at 1770–1900 m, fluvial Bidahochi-Richville aggradation to 6 Ma Aggradational Paleopro le Dated basalt mesa B Terraces from vill e (inverted paleochannel) Rice, 1977 >2200 m, Fence Lake Formation at 2440–2286 m, 6–8 Ma Hopi Buttes basalts (projected), 6.8 Ma 2800 Incision Point C 22-2.4 6.9 D (Flattop) to 1.7 Ma basalt mesas (inverted paleochannels) in headwaters. Two Ma White Mesa (Table 2)-Age in Ma X = Basalt with age (Ma) 2700 alluvium (WM) shown at correct elevation and gradient (green line), but inferred paleoconflu- Springer Fence Lk

alls ence was near modern Moenkopi Wash (red line). Incision point #3 (2 Ma Blue Mesa tuff) is 2600 alls Chuska SS. projected from Moenkopi Plateau to correct height above Moenkopi Wash (Fig. DR-1.2 [see foot- 2500 inslow 34-27 Ma St. Johns Cameron

W 18-6.0 Moenkopi Holbrook note 1]). New Ar-Ar dates on 0.89 Black Point basalt (incision point 5) and 0.34 Ma Tappen Wash Black F 2400 Grand F Wash 16-6.8 Spear Fm. basalt (incision point 7A) provide age estimates for Cooley et al. (1969) and Rice (1974) Wupatki 40-34 Ma

2300 ibab Monocline 11B terraces. (B) Differential incision in different reaches: steady 6–2 Ma incision of 20–40 m/Ma BM Eagar Fm. 2200 11A in the headwaters and central reach (both N and S of the LCR) based on eastern LCR tribu 3.67 55-34 Ma 0- taries (orange points), headwater basalt (black dots), average Hopi Buttes eruptive surface (red East Ka 2

) 2100 square), and Chevelon Butte (#14 yellow triangle), respectively. Post–2 Ma acceleration of rates 2B-2.0 X 7.8X 19-6.03 Cretaceous Strata WM 6.6 10 6 Ma K 2000 7.4 X to 160–170 m/Ma is seen within and below the LCR knickpoint (yellow diamonds). 7.0 X 6.6X 6.9 X 7.0 Fluvial Bidahochi Chinle 8.2 X 7.8 X 6.6xx 2 Ma c 1900 5 Ma X 7 7.7 X Lacustrine Bidahochi 23-1.98 Moenkopi

ation (m 24-1.6 9B TR m 1800 9A-15.19 17-6.52 Kaibab/Toroweep G 8A 8A-15.15 Ma 2 15 Ma 27-0.3 Pkt Elev 2A-2.0 213m 8B Coconino Sandstone 1700 15 Pc 1A -Cooley 1 Ma terraces Supai/Hermit Shale If so, this date would indicate the CR level was at 1290 m at 2.68 Ma (point 1B, 1600 137m 0.5 Ma IPPs/h 3-2.0 2 Ma 4-1.0 75m Fig. 14A), and the 2 Ma paleoprofile was at ~1200 m. However, this cave is in 1500 7 54m Mr Redwall Limestone 6A-0.8 26m a tributary; so the incision rate and magnitude are maxima (Table 1; Karlstrom 1400 5-0.89 C Mauv Limestone 7A-0.34 m 1300 Cba Bright Angel Shale et al., 2008; Crow et al., 2014). Nevertheless, an elevation of ~1200 m for the 1B-2.68 Ct Tapeats Sandstone 1200 2 Ma confluence is close to the estimate of ~1145 m obtained by assuming that 1100 the measured Quaternary rates of 160 m/Ma (documented over the past 623 ka; Crow et al., 2014) may have been semi-steady back to 2 Ma (Fig. 3). 1000 Blue Springs pC Precambrian 1C-0.625 basement 900 Enlarged About 4 km downstream from the confluence along the mainstem of the Pro le of Fig. 16 800 Colorado River, a bedrock strath 70 m above the river was dated at 623 ka by 0100 200300 400 500 600 Crow et al. (2014) yielding a bedrock incision rate of 147 m/Ma (assuming a km above Colorado River conuence depth to bedrock of ~21 m). This rate was refined by strath-to-strath dating from terrace flights between river miles 57 and 65, both above and below the 700 LCR confluence, that indicate a semi-steady average bedrock incision rate of Mean Hopi ButtesHopi Buttes B 160 m/Ma over the past 623 ka (Crow et al., 2014) placing the CR-LCR conflu- 600 Lower LCR Central LCR ence at 985 m elevation at 1 Ma (Fig. 3) and 925 m at 0.623 Ma (point 1C of Fig. Upper LCR Eastern Tributaries 14A and Table 1). regression (Upper LCR) regression (Eastern Tributaries) 500 mean Hopi Buttes basalts Lower and Central LCR

) In the central reaches of the LCR, above the LCR knickpoint near Cameron, 400 /Ma 40 m long-term post–6 Ma bedrock incision rates can be estimated based on the posi- CR ~ White Mesa 14-Chevelon Butte central L tion of the 8–6 Ma eruptive surface for the Hopi Buttes basalts. This was a long- ation (m alluvium N- 300 3- Blue Point tu lived, low-relief surface (Hopi Buttes surface of Cooley et al., 1969). Many of the Elev m/Ma 3 CR 23 8–6 Ma eruptions formed maars where lavas interacted with groundwater in central L S- what was likely the bottom of the LCR valley. This eruptive surface has been a 13A 22 12B 12A 200 ower variably dissected; it is essentially intact in the northeastern parts of the Hopi 5 13B s 41 m/Ma er Buttes (elevation ~1900 m), where it is marked by thin, far-traveled flows that CR and L 6B CR 170 m/M 4 L LCR headwat Petried Forest Maar interfinger with sediments of the upper (fluvial) Bidahochi Formation. Farther a 100 6A /Ma (far from LCR) southwest, toward the modern LCR, the volcanic field offers exposures of maar 7 Tributaries 23 m 100 m/M Eastern volcanic features that developed as much as 300 m below the eruptive surface ville ow (above knickpoint) 12C Springer (White, 1991; Dallegge et al., 2001). Calculated rates for individual dated basalts 0 12 3456798 (Table 1) are interpreted as maximum denudation rates relative to the modern Age (Ma) LCR as they are at large distances (18–75 km) from the modern river and lack

paleo-LCR gravels. However, assuming a laterally extensive low-relief surface, and may constrain the 46–61 m terrace (L-3B of Cooley et al., 1969) to be ca. LCR valley denudation rates range from 31 to 81 m/Ma (Table 1). Coliseum maar 350 ka and, together with the Black Point incision constraint, suggests that this is within 18 km of the LCR and gives a denudation rate of 50 m/Ma, which is region, like the CR, has undergone semi-steady average bedrock incision of similar to the rate of 65 m/Ma calculated using the average age (7.2 Ma) and ~170 m/Ma over nearly the past one million years. height (470 m) of the reconstructed eruptive surface (Table 1; #10). Shadow Mountain basalt flows give an40 Ar/39Ar isochron date of 280 ± On the Mogollon slope southwest of the LCR (Fig. 1), Holm (2001a) used 50 ka (Conway et al., 1997). The elevation of the lowest flow base is 50 m above basalt-capped surfaces in tributaries to postulate average incision rates of the level of Moenkopi Wash (Fig. DR-1.2 [see footnote 1]), which gives a maxi ~100 m/Ma (33–120 m/Ma) over the past 6 Ma. The best individual rates are mum tributary incision rate of 179 m/Ma, similar to the mainstem rate of 167 from basalts closest to the LCR. These give average rates of 51 m/Ma since m/Ma from the Tappan flow over the same ~300 ka time interval. Rice (1980) 6.41 Ma (Chevelon Butte; #14), 107–115 m/Ma over the past 1.92–1.63 Ma (East mapped basalt-bearing gravels and tephra that were derived from Shadow Sunset Mountain; #13), and 148 m/Ma over the past 0.81 Ma (Woodhouse Mountain in Hopi Trail Wash within 2 km of the LCR confluence; these project Mesa; #6). These data suggest increasing incision rates through time in the 70 m above the LCR and would give an incision rate of 250 m/Ma using the past 6 Ma (Fig. 14B). newer ca. 0.28 Ma 40Ar-39Ar age of Conway et al. (1997), whereas Rice (1980) Along Moenkopi Wash, a tributary north of the LCR, new dating of ashes reported 113 m/Ma using an older 0.62 Ma K-Ar date on Shadow Mountain can be used to calculate tributary incision rates. The 1.993 ± 0.002 Ma ash in (Damon et al., 1974; see section DR-1 [see footnote 1]). a deposit of White Mesa alluvium on top of Moenkopi Plateau (#3 of Table 1; Hereford et al., 2016) yields a tributary incision rate of 120–155 m/Ma when the Headwater Region planar tread of that deposit is projected along its slope to Moenkopi Wash (Fig. DR-1.2 [see footnote 1]). In Blue Canyon at the edge of Black Mesa (#4), an ash Figure 15 shows a geologic map of the headwater region of the LCR at correlated to the ca. 1 Ma Bishop Tuff was found in the L2B deposit. Projecting the eastern edge of the Springerville volcanic field. Basalt flows from volcanic the tread of that deposit to Moenkopi Wash yields a similar incision rate of fields have “run-out” geometries suggesting they followed paleodrainages, 120–125 m/Ma. The similar rate of ~120 m/Ma over the past 2 Ma for tributaries and they now stand as topographically inverted elongate mesas. Results pre- to the north and south of the LCR implies this rate for the mainstem LCR and sented here are preliminary as only a few of these mesas have gravels mapped was used to reconstruct the ca. 2 Ma paleoprofile in the central reaches of beneath the flows, and ages are K-Ar ages that are rarely from the optimum Figure 14A. locations to calculate incision rates. Figure 16 shows the base of basalts pro- The new 40Ar/39Ar date of 891 ± 13 ka for the Black Point flow (#5) agrees jected into the LCR profile, and red stars show incision rate control points listed with an independent 40Ar/39Ar date from Hanson (2007) of 873 ± 8 ka. The Black in Table 1. A 6.8 Ma basalt caps thin beds of Fence Lake and/or Bidahochi grav- Point flow originated in the San Francisco volcanic field and flowed into the els at Flat Top (#16), and a nearby 6.03 Ma basalt overlies river gravels; both LCR channel (Fig. DR-1.3 [see footnote 1]). Rice (1980) reported LCR gravels indicate long-term average incision rates of 40–45 m/Ma for the headwater re- atop the flow, and drill data indicate it overlies 22 m of LCR gravel. Gravels gion (Embid, 2009). However, another 6.52 Ma flow is at a considerable lower overlie a bedrock strath 151 m above the modern river (Fig. DR-1.4 [see foot- elevation relative to the LCR (#17) and gives a lower incision rate of 31–38 note 1]; Haines and Bowles, 1976) giving a bedrock incision rate of 193 m/Ma, m/Ma relative to the LCR. Incision rates of ~23 m/Ma are obtained from ca. somewhat higher than the 160 m/Ma post–623 ka rate at the CR-LCR con- 6 Ma basalts in eastern tributaries to the LCR. Figure 14B shows considerable fluence. The Black Point flow age constrains the 122–152 m terraces (L-2B of post–6 Ma incision rate variation in the headwaters with rates of 10–50 m/Ma. Cooley et al., 1969) to be ca. 1 Ma. The Bishop tuff at Blue Canyon is in gravel Post–6 Ma flows closest to the modern LCR show a general relationship of the L-2B surface, which provides another direct date of L-2B. This surface where younger flows are inset into older flows (Fig. 16). Based on its paleo- extends downstream as far as Tuba City and is projected to Moenkopi Wash channel-like geometry, we infer that the 3.67 ± 0.12 Ma Coyote Wash flow in Figure DR-1.2 (see footnote 1) to give heights of 118–126 m above Moenkopi marks the paleochannel of the LCR, although we have not mapped gravels Wash and corresponding tributary incision rates of 118–126 m/Ma. at the base of the flow. The lobe shows an uneven height above the modern Tappan flow (#7) was mapped in detail by Rice (1977). It flowed >50 km river profile, perhaps compatible with a graben between the Coyote Wash fault down Tappan Wash, then thickened dramatically (from 15 to 30 m), filled and inferred faults (Embid, 2009). The height of the base of the flow at Lyman the steep-walled LCR paleochannel, and flowed both upstream (1.6 km) and Lake is 1950 m, giving an incision rate of 37 m/Ma (Fig. 16A), similar to average downstream (~11 km) within the earlier LCR river course (Fig. DR-1.3 [see foot- post–6 Ma rates. note 1]). The new 40Ar/39Ar date on basalt from within a meter of the base of the A prominent but higher flow lobe is called Black Mesa; it has the 2.46 ± flow is 339 ± 5 ka (Fig. 11D). At this location, Tappan flow overlies LCR gravels 0.04–2.37 ± 0.03 Ma Mesa Parada flow possibly inset on its northeast edge (Fig. that sit on a bedrock strath 57 m above the modern LCR channel giving an 15). The flow(s) appear to have originated in the Red Hill–Quemado area to the incision rate of 167 m/Ma. This sample is from just above the LCR knickpoint east and filled a paleovalley cut into older basalts (McIntosh and Cather, 1994).

Explanation: 109°30′ 109°20′ 109°10′ 109° 18-6.03 Location and age (Ma) of TR dated basalt ows keyed to TR Tbi Table 1 incision rates Tbi Carrizo Wash Tbi Fault Line Qg4 TR Qg5 Anticlinal trace Tbi K Qg4 Qg5 TR Synclinal trace TR Qg4 t TR t Travertine Sample Qg4 Qg3 Tb Location St. Johns Qg3 R Map Units: 34°30′ T Alluvium and colluvium Tg2 t t t Mesa The Buttes 22-2.37 25-1.56 Tg3 Parada t Travertine deposits 17-6.52 Black Mesa Tg3 Salado t Tbi Qg5 Gravel terrace 5 Springs t K Coyote Wash flow Qg2 t t Qg4 Gravel terrace 4 Qg2 Figure 15. Generalized geologic map of Lyman the Springerville area (modified from 1.98 Embid, 2009). The Little Colorado River in 1.67–0.00 Ma Basalts Lake 34°20′ this region cuts through the Plio-Pleisto- Tg1 cene Springerville volcanic field. Incision Little Colorado River t data points are numbered (referenced to Qg3 Gravel terrace 3 Tr Qg2 Table 1). TR K Richville Formation K Tr (Quemado) Lower Pleistocene 26-0.75 TR Qg2 Gravel terrace 2 24- 1.67 t Te 19-

The Buttes 6.03 8.97–1.68 Ma Basalts Ceda r Mesa fault / anticline K Gravel terrace 1 21B- 20- Tg1 34°10′ t (>2 Ma?) 2.94 3.67 Te V Bidahochi Formation T19 Coyoteernon faul Tbi T21 Springerville Qg5 (4–6 Ma) T40 Qg5 Qg3 T38 T23-26 Flat Top W 21A-3.06 Eagar Formation T28 T37 asht flow Te Qg5 Qg3 (Eocene-Early Oligocene) Te 16- 6.8 Lyman Te K Cretaceous Formations LCR Lake N 0510 Triassic Formations TR 18-6.03 (Chinle and Moenkopi) lt

Explanation: 350 Basalt ow base & top 3.0 16-6.80 Location of dated basalts 300 2 =0.54 22-2.37 Mesa Parada 18-6.03 R ow inset into Base of ow 2.9 Black Mesa 2 =0.91 without #21,22 250 R 42 Incision rate location (and rate m/Ma)

) 19- 6.03 age assumed Lobo Knoll for Black Mesa River prole 2.8 200 17-5.31younger age used Incision point (Table 1) (instead of 6.52) 16-6.80 ~41 m/Ma & age (Ma) ong-term steady incision 2.7 150 L Flow ll 23-1.98 0.8 Scraper 20-3.67 Fault Kno 2.6 Strat Height (m 100 25-1.56 1.67 Ma Gravels 19- 24-1.67 16- 18- 6.03 6.80 6.03 2.5 50 26-0.75 Springerville ow Flat Top 21-3.06 & 2.94 0 2.4 12354678 Age (Ma) 44 45

2.3 20-3.67 T Eagar 2.2 21-3.06 Elevation (km) 2.1 Black Mesa 19-6.03 20-3.67 Coyote Wash 26-0.75 TR Chinle 2.0 23-1.98 41 issure Vent 24-1.67 17-6.52 F 33 IP San Andres 60 36 1.9 37 LL 18- IP Glorieta 38 1.8 31- ult

38 t ult

sh fault sh Wa faul

erred fa IP Corduroy te te

1.7 inf

cho dar Mesa fault Mesa dar ernon fa n

Ce Coyo Co 1.6 450470 490 510 530 550 570 km above Colorado River confluence

Figure 16. Paleoprofiles in the upper Little Colorado River (LCR) region (modified from Embid, 2009) showing 6.8–0.8 Ma basalt flows and basalt incision points in the upper LCR region. Elevations of bases of flows (dark lines) and tops (light shading) are projected into the LCR profile. LL—Lyman Lake. Numbers, ages of flows, bedrock incision magnitudes (vertical lines), and calculated incision rates (m/Ma) are keyed to Table 1. Inset shows a plot of basalt age versus height of the base of the flow above the modern river and indicates semi-steady incision at 41 m/Ma.

Black Mesa flows arc around St. Johns Dome and fill a channel cut in Bida- provisional LCR incision rate of 41 m/Ma using the 6.03 Ma date, whereas the hochi gravels from 2290 to 1980 m elevation at the eastern edge of the study local incision rate from sample #37 to Coyote Creek is 35 m/Ma. area. The higher projected height of this flow relative to the younger Coyote A regression of the basalt data along the LCR from south of Springerville Wash flow (Fig. 16) is more compatible with using the 6.03 Ma date to the SE to St. Johns (inset to Fig. 16) suggests a semi-steady incision rate of 41 m/Ma (#19 and #37) rather than the 2.37 Ma Mesa Parada date to estimate incision from 6 to 1 Ma. Outlier points may reflect complex variation expected in inci- rates. The NW tip of Black Mesa is 250 m above the LCR and would yield a sion rates across paleo-knickpoints (#21) and uncertain dating of flows (#22).

A similar incision value of 41 m/Ma is obtained from Largo Creek–Carrizo Wash INTERPRETATION OF POST–6 Ma DIFFERENTIAL INCISION tributary to the LCR based on a 2.46 Ma basalt flow 100 m above the creek OF THE LCR (Love and Connell, 2005). Basalt data from eastern tributaries of the LCR (#35– 44 of Fig. 1 and Table 1; McIntosh and Cather, 1994) show somewhat slower Figure 14 provides a summary of the differential incision history in both but still semi-steady incision rates of 23 m/Ma over the past 6 Ma. Thus, given space and time for the LCR. Basalt data from the upper reaches of the LCR available dating and projection of basalt surfaces, we interpret the headwater and its tributaries suggest semi-steady incision at rates of 30–40 m/Ma from reaches and its tributaries of the LCR to record semi-steady bedrock incision of ca. 6 Ma to 300–100 ka. This rate is similar to slow denudation rates well away ~20–40 m/Ma from 6 to 1 Ma. from the LCR at Anderson Mesa. Central reaches of the LCR give higher aver- Figure 15 shows gravel terraces that were mapped by Sirrine (1958), and age rates from 6 Ma to present. For example, the incision magnitude since the Figure 17A projects gravel straths into the LCR profile (Embid, 2009). The data formation of the Hopi Butte eruptive surface at ca. 7.2 Ma is ~470 m, yielding are preliminary and would benefit from additional terrace mapping, correla- a long-term average incision rate of 65 m/Ma. However, our new data suggest tion, and dating of gravels, for example with detrital-sanidine methods. Never this long-term rate is best interpreted in terms of slow 30–40 m/Ma rates from theless, younger terraces are shown to be inset into older basalts and older 6 to 2 Ma (similar to upper reaches) followed by accelerated incision in the past terraces supporting the general notion of progressive incision since 6 Ma. 2 Ma. Thus, inferred paleoprofiles from 6 to 2 Ma in Figure 14A show relatively Remnants of the oldest gravel (Tg1) are inset into Eagar Formation along slow incision between 6 and 2 Ma. Coyote Creek at elevations of 2165–2195 m. Up to 15 m thicknesses of Qg2 The new 40Ar/39Ar sanidine age of ca. 2 Ma for White Mesa alluvium both on gravel underlie much of the Springerville volcanic field at elevations 60–100 m the Moenkopi Plateau (Blue Point tuff) and in the White Mesa paleovalley gives above the LCR riverbed. Based on their heights and comparison to dated ba- incision rates of 137 m/Ma (range of 119–155 m/Ma) for Moenkopi Wash over salts, Tg1 is likely >2 Ma, and Qg2 is ca. 1–2 Ma. Projections of Tg1 and Qg2 the past 2 Ma. This wash has its modern confluence with the LCR just above suggest convexities that are similar in shape to the modern knickpoints. Qg3 the LCR knickpoint, and we infer that its paleoconfluence at 2 Ma was also gravel (3–5 m thick) is inset ~10 m below Qg2 and is preserved in Big Hollow above the knickpoint (Fig. 14A). Tributaries typically incise at the same rate as Wash beneath 1.68–1.87 Ma basalts. Undated travertines along Carrizo Creek the mainstem at their confluence, and we note that there were similar average are deposited on this surface. incision rates for tributaries north (Moenkopi Wash at 137 m/Ma over 2 Ma) Uranium-series dates on travertine platforms, mound spring complexes, and south (East Sunset Mountain relative to East Clear Creek at 115–120 m/Ma and on travertine-cemented gravels in the LCR headwater region near Lyman over 1.92–1.63 Ma) of the LCR. We interpret this to indicate that the average Lake also provide incision constraints. Figure 17B shows that platforms and incision rate of the central reaches of the mainstem LCR over the past 2 Ma was mounds extend to variable elevations in the landscape due to artesian spring also 115–140 m/Ma and hence ~3 times faster than the 6–2 Ma rates of 30–40 accumulations (Embid, 2009). However, where travertines cement gravels in m/Ma in the headwaters. strath terraces, travertine ages show semi-steady bedrock incision rates of 38 Post–1 Ma rates were perhaps still higher: 122 m/Ma for the Blue Canyon m/Ma from 300 to 100 ka and increased rates since then (Fig. 17C). The oldest tuff (#4), 170 m/Ma in the mainstem at Black Point basalt (#5), and 148 m/Ma at dated travertine sample that overlies a strath is 304 ± 13 ka (#27) and was Woodhouse Mesa (#6). Youngest incision points give rates of 160 m/Ma over sampled from the southern end of the Salado travertine platform at an ele- 625 ka in the mainstem CR (#1C) and 167–179 m/Ma over 300 ka in the LCR and vation of 1827 m; it overlies 1 m of gravels that in turn rest on a strath 26 m Moenkopi Wash tributary (#7A and #7B). The overall data suggest an increase above the LCR giving an average incision rate of 87 m/Ma. A sample (#30) in incision rates from 40 to 170 m/Ma in the lower and central LCR over the past 5.5 m higher in the travertine platform gives an age of 272 ka and an incision 2 Ma (upper curve in Fig. 14B). estimate (from the same strath to the river) of 109 m/Ma (Fig. DR-1.6 [see footnote 1]). A nearby mound next to Lyman Lake dam (#28) of 281 ± 15 ka formed 60 m higher atop river gravels on a well-exposed strath; this sample gives DISCUSSION: POSSIBLE DRIVERS OF POST–2 Ma an incision rate of 153 m/Ma. Sample #29 is from the northern end of Lyman ACCELERATION OF INCISION Lake, and it gives an age of 255 ± 6 ka and an incision rate of 94 m/Ma (Fig. DR-1.5 [see footnote 1]). Sample # 31 gives an age of 224 ± 3 ka with a strath at Tributaries are required to keep up with the incision rate of the mainstem 30 m and an incision rate of 134 m/Ma (Fig. DR-1.7 [see footnote 1]). Samples (or form a hanging valley) such that incision rates at the confluence of the LCR of ca. 100 ka travertines, #32–34, are all from the bases of individual mounds and CR are assumed to have been the same, and hence incision rates in east- between Lyman Lake and Salado Springs overlying river gravel and/or strath ern Grand Canyon may also have accelerated in the past ~2 Ma. This section contacts and are dated 101 ± 1, 97 ± 1, and 78 ± 2 ka, respectively. They show discusses possible driving forces that may help explain the post–2 Ma in- that the average river incision rate in the past 100 ka increased to ~278 m/Ma crease in incision rates in the lower LCR and perhaps in eastern Grand Canyon. (average of three data points). This discussion includes a summary of recent models for the Grand Canyon

2.6 2.5 A 2.4 Qg3 18-6 Ma m 2.3 i F 20-3.87 Ma ch Tg1 ho da a Bi Te 4-6 M 2.2 Qg5 19-6.03 Ma Te 2.1 Qg2 Ku

21-2.94 Ma TR Chinle 2.0 R 23-1.98 Tc Explanation: IP San Andres Elevation (km) Qg3 1.9 Qg5 Projected straths from 24-1.67 TRc Qg4 Coyo Sirrine (1958) gravelsIP Glorieta 1.8 Qg5 Ce dar Mesa faul te IP sa Little Colorado River pro le

d fault Wa fault

e ault

r f

sh faul o Projected base of basaltsIP Corduroy IPg er h 20-3.67 Ma

1.7 rnon

inf Ve

Co t IP c Fault 1.6 t 450 470490 510 530 550570 km above Colorado River confluence 60 26- youngest basalts 20-3.67 C B 50 a m/M 23-1.98 24-1.67 y=38.3 ) R=0.78 40

Lyman Lake 30 33 24 30 # ka m/Ma Height (m ) 32 34 Elevation (m 29 27 27 0.304 87 20 LCR 28 0.281 153 Salado 29 0.255 94 Springs 30 0.239 109 31 0.224 134 10 = travertine 32 0.101 257 33 0.097 309 mound and ori ce 34 0.078 269 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 km above Colorado River confluence Age (Ma) Figure 17. (A) Strath heights for gravel terraces mapped by Sirrine (1958) projected into the plane of the Little Colorado River profile (modified from Embid, 2009). Dashed lines are base of dated basalts (from Fig. 16) for comparison. Strata below the profile are: Te—Tertiary Eagar Formation; Ku—Cretaceous rocks undivided; TRc—Triassic Chinle Formation; Psa— Permian San Andres Formation; Pg—Permian Glorieta Formation; Pc—Permian Corduroy Formation. (B) Heights and ages of dated travertines that cement river gravels near straths. (C) Travertine-based incision rates through time over the past 300 ka as compared to rates from youngest basalt flows.

region that variably emphasize tectonic, geomorphic, and/or climatic drivers Uplift Associated with Magmatism in the Hopi Buttes for landscape evolution. Multiple forcings are probable, but their relative im- and/or Springerville Volcanic Fields portance remains in debate; thus this section also includes ideas about future work to test alternate hypotheses. Headwater uplift and base-level fall are difficult to distinguish geomorphic- ally, and both cause changes in river gradients. The profile of the LCR shows Mantle-Driven Uplift a major knickpoint in its lower reaches that suggests base-level fall, but headwater plateau-like uplift may have caused a downstream knickpoint and higher Epeirogenic uplift of the southwestern Colorado Plateau relative to the incision rates to form at the edge of the uplifted area, similar to other examples Basin and Range (Karlstrom et al., 2008) and of the Rocky Mountains rela- (Whipple, 2004; Rosenberg et al., 2014). tive to the Colorado Plateau (Karlstrom et al., 2011) in the past 10–6 Ma may Several studies have suggested that rivers are responding to surface uplift have been driven by changes in mantle buoyancy. Moucha et al. (2008, 2009) associated with a NE-trending zone of Quaternary magmatism of the Jemez invoked large-scale mantle convection. Van Wijk et al. (2010) invoked edge- lineament that extends across New Mexico from the Springerville volcanic driven smaller-scale mantle convection. Levander et al. (2011) invoked litho- field to the Great Plains (Wisniewski and Pazzaglia, 2002; Nereson et al., 2013; spheric delamination and asthenospheric return flow. Crow et al. (2011, 2014) Channer et al., 2015). Rivers that cross suborthogonally across the Jemez lin- invoked a migrating zone of mantle modification recorded at Earth’s surface eament have convex profiles, convex terrace flights, and tilted basalt paleosur- by an inboard sweep of basaltic volcanism. However, the timing, scale, and faces that are interpreted by these studies to reflect several hundred meters mechanisms, as well as the existence (Pederson et al., 2013), of post–10 Ma of broad epeirogenic doming above the lineament. The Springerville volcanic mantle-driven surface uplift remain controversial. field has been volcanically active for the past 10–6 Ma, and the Hopi Buttes Most mantle uplift models propose regional-scale (western United States) field was active 8–6 Ma such that the turnaround from aggradation to incision uplift. The Crow et al., (2014) model is of most appropriate time frame, loca- after 6 Ma and the semi-steady 40 m/Ma incision rates since then might reflect tion, and spatial scale to explain the acceleration of incision in the LCR region. magmatically driven uplift. Springerville has also had voluminous basaltic vol- As shown in Figure 18, progressive NE-migration of a sharp velocity contrast canism in the past 2 Ma; but present data suggest that the Springerville head- along a step in the lithosphere-asthenosphere boundary that is now located water reaches have had semi-steady incision from 6 to 0.3 Ma with no marked near Lees Ferry may be manifested by a trailing sweep of basaltic volcanism increase in incision at 2 Ma, whereas this acceleration seems to be localized that is progressively becoming younger and more asthenospheric toward the in lower reaches. Plateau center (Crow et al., 2011). In this model, the sweep of volcanism itself can uplift Earth’s surface due to constructional topography (e.g., San Francisco Passage of the Lees Ferry Knickpoint in the Grand Canyon Peaks reach 3.8 km in elevation), and the asthenospheric upwelling that is driving the volcanism may add buoyancy that adds several hundred meters of Geomorphic explanations for differential incision invoke transient knick- long-wavelength doming and differential surface uplift. The latter was posited points (Darling et al., 2012; Donahue et al., 2013; Aslan et al., 2014). Several to be driving the observed higher incision rates in the past 0.6 Ma in eastern studies have proposed passage of a transient wave of incision in the mainstem Grand Canyon relative to central and western Grand Canyon (Crow et al., 2014). CR that now forms the regional-scale knickpoint at Lees Ferry (Pelletier, 2010) Figure 18 expands this concept, adds the lower LCR region of higher river in- or perhaps has migrated to Cataract Canyon (Fig. 2; Cook et al., 2009). Cook cision (dashed oval), and suggests that the Mesa Butte fault system, which et al. (2009) noted that the LCR and other CR tributary channel profiles are the San Francisco Peaks are in part built along, may be a zone that is focusing downturned in their lower reaches with knickpoints 150–200 m above their NE propagation of large-volume volcanism toward the large-velocity contrast confluence, consistent with a transient that has passed the CR-LCR confluence centered under Lake Powell. Increased subsurface magmatic flux and mantle in eastern Grand Canyon. Their modeling explored the incision rate changes buoyancy may have started passing beneath the CR-LCR confluence region at expected at a given location due to passage of a knickpoint, and they modeled ca. 2 Ma. This is consistent with NE migration of volcanism in the San Francisco a scenario in which the Colorado River in the Glen Canyon region underwent volcanic field at a rate of 30 km/Ma in the past 2 Ma (Tanaka et al., 1986). Surface a pulse of rapid incision in the past ~500 ka. Darling et al. (2012) suggested doming would need to have been on the order of 100–200 m over the past 2 Ma incision in the Glen Canyon region has accelerated in the past 300 ka. Abbott to explain the four-fold difference in incision magnitude (and rates) of 60–80 m et al. (2015) invoked the Cook et al. (2009) model and suggested passage of an (30–40 m/Ma) in the LCR headwaters versus 280–320 (140–160 m/Ma) in the incision wave through central Grand Canyon between 500 and 400 ka. How- lower reaches. We assume that river incision would have kept pace with uplift ever, such recent timing is not compatible with Grand Canyon incision data for (Braun, 2010), likely by steepening of the profile, and that isostatic response (re- semi-steady incision (Crow et al., 2015). The Cook et al. (2009) modeling (their bound) to the ~220–240 m of differential denudation since 10 Ma might account fig. 12) suggests that incision rates would peak and then decline dramatically for nearly half (~100 m) of the differential uplift (Lazear et al., 2013). at any given point following passage of the transient. However, existing data

Explanation 114° W 113° W 112° W 111° W 110° W 109° W >0.2 Ma VP (%) at 80 km 38°N 3.5 km/Ma Incision Rates (Schmandt and (m/Ma) Humpreys, 2010) Escalante 2 km/Ma *# 4–50 High :2

*# *# 50 –100 San Juan *# 100–200 *#*## # *#*#*# # *# >200 37°N Low:–2

o Age of Volcanics (Ma) from NAVDAT

Colorad 0–1.25 5 km/Ma # *# *#*# 1.25 –2.5 *# # *# *# *#*#*#*# *# ## 1 2.5–5.0 36°N*# *# *# * 5.0–7.5 *# *# # *# *# #7 3 7.5–10.0 *#5 10.0 –15.0 *# 15.0 –20.0

20.0 –25.0 Little Colorado Spacial trend and rate of 8 km/Ma 35°N migrating volcanism 5 km/Ma *#*# Quaternary faults *# (gray) *#*#*# 34°N # 0315 06090120 km

Figure 18. Model for onset of rapid incision in the Little Colorado River–Colorado River (LCR-CR) region (dashed black oval) due to magma migration continuing in the subsurface NE of the San Francisco Peaks (white dashed line). Diagram (modified from Crow et al., 2014) shows relative mantle P-wave velocity (VP) at 80 km (from Schmandt and Humphreys, 2010) and migration of basaltic volcanism (colored symbols from North American Volcanic and Intrusive Rock Database [NAVDAT]).

in the CR for semi-steady incision over 625 ka in eastern Grand Canyon, and Bedrock-Controlled Knickpoints ~4 Ma in western Grand Canyon, and a similar steady incision rate of 160–170 m/Ma in the mainstem of the lower LCR based on the 0.34 Ma Tappan and The Lees Ferry and LCR knickpoints are both located near the Kaibab 0.89 Ma Black Point basalt flows do not support the modeled changes in in- Limestone– Moenkopi Formation contact. The steeper parts of the river pro- cision rates expected from passage of a transient knickpoint in the CR or LCR files are in more resistant carbonate-dominated strata of the Paleozoic section within the past ~1 Ma. Instead, incision data favor a longer-term, steady-state below the knickpoint and shallower gradients are observed in more erodible process such as tectonic uplift. sandstone-shales of the Triassic section above (Figs. 2 and 14). Cook et al.

(2009) proposed that the arrival of an incision transient at the Lees Ferry up- followed by cooling temperatures since ca. 2.7 Ma. This has been proposed to stream-dipping lithologic knickpoint may have initiated a pulse of incision to have led to an intensification of glacial climates (Haug et al., 2005) accompa- sweep rapidly (400 m/Ma) through the upper weaker rocks until it encoun- nied by increased seasonality of climate and higher erosivity of geomorphic tered stronger rocks far upstream in Cataract Canyon. Similar models for rapid systems, perhaps globally (Molnar, 2004; Molnar et al., 2006). At shorter time knickpoint propagation through weak units were proposed for the Mancos scales, oscillating Quaternary glacial-interglacial conditions have caused cyclic Shale by Darling et al. (2012) and Aslan et al. (2010, 2014). Donahue et al. (2013) changes in bedrock incision versus aggradation recorded by river terraces (cf. posited that highest incision rates within the harder basement rocks of the Bull, 1991; Hancock and Anderson, 2002; Pederson et al., 2006). For time peri- knickpoint region of the Gunnison River relative to slower rates in upstream ods of <150 ka (e.g., Karlstrom et al., 2013b; Pederson et al., 2013), the terrace and downstream reaches underlain by erodible Mancos Shale were evidence record may not have averaged out glacial-interglacial oscillations and hence that the Gunnison knickpoint is a young transient feature of the river system. may give maximum bedrock incision rates. Darling and Whipple (2015) expanded the discussion of knickpoints, tribu- The effects of climate changes on incision rates and patterns in the Rocky tary profile patterns, and bedrock strength to explain profile patterns that form Mountain– Colorado Plateau region are not well documented but would be when tributary drainages have easily eroded strata overlying stronger strata. expected to have regional effects rather than the observed sub-regional inci- They noted that knickpoints form due to more rapid erosion within the can- sion variations (e.g., Nereson et al., 2013; Rosenberg et al., 2014). The 6–5 Ma yon than on the surrounding lithologically controlled, low-relief benches. But, climate changes seem likely to have influenced the change from aggradation similar to their conclusion for the Hualapai Plateau of westernmost Grand Can- to erosion ca. 6–5 Ma in the Southwest and the integration of the Colorado yon, the region above the LCR knickpoint is an older and long-lived stripped River system. Whether climate change was the main driver or interacted with surface (e.g., Childs, 1948), not a lithologically controlled bench. Importantly tectonic influences requires additional data on spatial variations and temporal for understanding the LCR knickpoint, there are similar incision rates above, incision variations and more precise dating of terraces and paleoriver deposits within, and below the knickpoint: Incision through Mesozoic strata at Black of this age. Climate change to more erosive climate at ca. 2.6 Ma is a possi- Point above the knickpoint has averaged 170 m/Ma over 890 ka; incision of the ble contributor to the acceleration of incision in the past 2 Ma in the LCR and Tappan flow at the LCR knickpoint has averaged 167 m/Ma over 342 ka; and eastern CR regions but is hard to reconcile with continued steady incision in incision in the mainstem CR near the confluence region has been semi-steady western Grand Canyon and the LCR headwaters since ca. 6–4 Ma and cannot over the past 625 ka at 160 m/Ma. Thus, we infer that the LCR knickpoint is a by itself explain the observed differential incision. semi-stable feature of the LCR profile that has been pinned (or hung up) at the Paleozoic–Mesozoic contact for most of the past 2–1 Ma (Fig. 14A) and reflects the increased gradient needed to carve Paleozoic carbonates at about the same CONCLUSIONS rate as Mesozoic strata. Additional strath-to-strath incision data in multiple locations are needed to test if the same explanation may be true for the Lees The incision and denudation history of the Little Colorado River region Ferry knickpoint. Our present view is that the prominent knickpoints at Lees over the past 70 Ma was punctuated by three denudation pulses, each linked Ferry and the LCR are bedrock-influenced knickpoints that resulted initially to river integration and/or canyon- carving episodes and coupled to pulses from headwater uplift and/or base-level fall that accompanied 5–6 Ma down- of cliff retreat. Post-Laramide denudation at 70–50 Ma stripped Paleozoic and ward integration of the CR and LCR through Grand Canyon, with amplification Mesozoic strata NE off the Mogollon highlands via cliff retreat. Mid-Tertiary of the knickpoint caused by renewed 2 Ma differential uplift. canyon carving of the LCR paleovalley and East Kaibab paleocanyon took place 25–15 Ma and caused the regional post–Chuska sandstone denudation of the Change toward Post-Miocene Erosive Climates southern Colorado Plateau. Post–6–5 Ma denudation was initiated by head water uplift and/or base-level fall and resulting downward integration of the CR There are several postulated changes in post-Miocene climate that may to the Gulf of California. Six to 5 Ma CR integration across the Vermilion Cliffs have influenced landscape evolution. A mid-Miocene warm period (17–15 Ma— at Lees Ferry allowed drainage from the central Colorado Plateau to flow south Zachos et al., 2004; or 17–12 Ma—Chapin, 2008) was followed by cooling tem- along the east side of the Kaibab monocline to a confluence with the paleo-LCR peratures since then (Fox and Koch, 2003). Chapin (2008) argued that opening and its East Kaibab paleocanyon (speculatively at ~1600 m elevation) near the of the Gulf of California by ca. 6.4 Ma resulted in an intensification of the North modern CR-LCR confluence. American Monsoon and increased erosion in the Southwest. This proposed Long-term post–6 Ma LCR average bedrock incision measured using dated climate shift may have combined with a global d18O ~2‰ negative seawater basalts shows that incision magnitude and average rate decrease upstream shift starting ca. 5.3 Ma (Zachos et al., 2004) that may reflect increased erosion. on the LCR: ~800 m (>130 m/Ma) at the confluence, ~450 m (~75 m/Ma) in A mid-Pliocene period (ca. 3 Ma) that was about ~3 °C warmer and more humid the Hopi Buttes area, ~240 m (~40 m/Ma in the LCR headwaters), and ~100 m than today (Thompson and Fleming, 1996; Haywood and Valdes, 2004) was 150–200 m (23 m/Ma) in the eastern LCR tributaries east of the Springerville

volcanic field. Most of this differential incision is attributed to uplift in the re- has arguably remained near sea level for the past 5 Ma (Crossey at el., 2015; gion of the CR‑LCR confluence in the past 2 Ma. but c.f. Spencer et al., 2013). Differential incision data suggest that the Colo- Perhaps the most surprising and impactful advance of this study (and in rado Plateau has gone up hundreds of meters relative to lower reaches during Hereford et al., 2016) is the discovery that the White Mesa alluvium and paleo this time (Karlstrom et al., 2007, 2008; Crow et al., 2014). Likewise, the Rocky valley are ca. 2 Ma, not a Miocene paleoriver (Lucchitta et al., 2013b), nor a Mountain region may have been uplifted several hundred meters relative to tributary to the fluvial ca. 6 Ma upper Bidahochi Formation. Instead this alluvial Colorado Plateau over the past 10 Ma (Karlstrom et al., 2011). Several regions system records a modest-size tributary to the Little Colorado River analogous above the Jemez lineament have been proposed to have been broadly uplifted to modern Moenkopi Wash. Due to its present gradient and high landscape over the past 5 Ma (Nereson et al., 2013; Channer et al., 2015). This paper pos- position, this young age requires that there was relatively little incision in the tulates surface uplift associated with the San Francisco volcanic field. We favor lower LCR valley between 6 and 2 Ma compared to more rapid incision of the a component of magmatically driven surface uplift as a mechanism to increase lower reaches of the LCR in the past 2 Ma. The acceleration of regional denu- river gradients and introduce knickpoints. Climate changes at 6–5 Ma and ca. dation and incision of the Little Colorado River region that started between 2.6 Ma are likely to have interacted with any tectonic changes to influence 2 and 1 Ma has beheaded the NE extension of the drainages on Black Mesa, regional river integration and incision rates but do not explain differential inci- dramatically deepened the region of the San Juan–Colorado River confluence, sion at the scales of the above examples. and carved much of the deep LCR gorge below Cameron. This study shows the importance of differential incision studies at regional New 40Ar/39Ar and U-series geochronology provides late Quaternary bed- spatial scale and long (several Ma) temporal scales and the power of combin- rock incision constraints throughout the LCR system over the past 6 Ma. Inci- ing thermochronologic and geologic data sets. Future progress and additional sion rate of the lowermost LCR has kept pace with that of the CR at the con- tests of competing models require “differential thermochronology” studies fluence over the past ~1 Ma that has averaged >160 m/Ma (Marble Canyon) to linked to additional strath-to-strath incision data at numerous localities along 167–193 m/Ma (lower LCR valley). In the middle reaches of the LCR, elevated the major rivers of the CR system. Precise dating of paleoriver deposits using basalt flows suggest that acceleration of incision started 2.0–1.5 Ma. Basalt a variety of techniques is needed, and detrital-zircon and sanidine dating com- constraints from headwater reaches and eastern LCR tributaries suggest that plement each other to constrain maximum depositional ages and provenance incision rates have been steady at 20–40 m/Ma for the past 6–0.3 Ma. Ura- of paleoriver deposits. Sanidine dating in particular is shown in this paper to nium-series dates on travertine that cements gravels directly above bedrock have led to a breakthrough in understanding Cenozoic evolution of LCR and straths suggest late Quaternary average incision rates of ~38 m/Ma from 1 Ma CR river systems of the Colorado Plateau–Rocky Mountain region. Another key to 100 ka and higher incision rates of 278 m/Ma in the past 100 ka. broader implication of this work involves quantification of differential uplift At regional scale, the ~40 m/Ma steady incision rate of the upper LCR is magnitude and spatial patterns that can provide input for geodynamic model- attributed to epeirogenic uplift of the entire Colorado Plateau. The 100 m/Ma ing of links between dynamic topography and drainage evolution. higher incision rate of the lower reach and LCR-CR confluence region is interpreted to reflect sub-regional epeirogenic uplift of the eastern Grand Canyon ACKNOWLEDGMENTS relative to western Grand Canyon as proposed in earlier papers (Karlstrom We acknowledge the decades-long contributions of Maurice Cooley on the Little Colorado River et al., 2008; Crow et al., 2014). The new data for acceleration of incision in the valley. Our research was supported by National Science Foundation grant EAR-1348007 to Karl- past 2 Ma in the lower LCR implies a similar change in the CR system, and we strom and Shuster. The paper benefitted from reviews by Andre Potochnik, Kelin Whipple, and propose surface uplift of 50–60 m/Ma near the CR-LCR confluence relative to Stuart Thomson. both the LCR headwaters and central Grand Canyon to explain incision variations. 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